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Yao
2024-12-20 17:49:45 +08:00
parent 86b0363ce1
commit 654d456c7d
7011 changed files with 1705926 additions and 7 deletions
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// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "absl/synchronization/mutex.h"
#ifdef _WIN32
#include <windows.h>
#ifdef ERROR
#undef ERROR
#endif
#else
#include <fcntl.h>
#include <pthread.h>
#include <sched.h>
#include <sys/time.h>
#endif
#include <assert.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <algorithm>
#include <atomic>
#include <cstddef>
#include <cstdlib>
#include <cstring>
#include <thread> // NOLINT(build/c++11)
#include "absl/base/attributes.h"
#include "absl/base/call_once.h"
#include "absl/base/config.h"
#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/atomic_hook.h"
#include "absl/base/internal/cycleclock.h"
#include "absl/base/internal/hide_ptr.h"
#include "absl/base/internal/low_level_alloc.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/spinlock.h"
#include "absl/base/internal/sysinfo.h"
#include "absl/base/internal/thread_identity.h"
#include "absl/base/internal/tsan_mutex_interface.h"
#include "absl/base/optimization.h"
#include "absl/debugging/stacktrace.h"
#include "absl/debugging/symbolize.h"
#include "absl/synchronization/internal/graphcycles.h"
#include "absl/synchronization/internal/per_thread_sem.h"
#include "absl/time/time.h"
using absl::base_internal::CurrentThreadIdentityIfPresent;
using absl::base_internal::CycleClock;
using absl::base_internal::PerThreadSynch;
using absl::base_internal::SchedulingGuard;
using absl::base_internal::ThreadIdentity;
using absl::synchronization_internal::GetOrCreateCurrentThreadIdentity;
using absl::synchronization_internal::GraphCycles;
using absl::synchronization_internal::GraphId;
using absl::synchronization_internal::InvalidGraphId;
using absl::synchronization_internal::KernelTimeout;
using absl::synchronization_internal::PerThreadSem;
extern "C" {
ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)() {
std::this_thread::yield();
}
} // extern "C"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace {
#if defined(ABSL_HAVE_THREAD_SANITIZER)
constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kIgnore;
#else
constexpr OnDeadlockCycle kDeadlockDetectionDefault = OnDeadlockCycle::kAbort;
#endif
ABSL_CONST_INIT std::atomic<OnDeadlockCycle> synch_deadlock_detection(
kDeadlockDetectionDefault);
ABSL_CONST_INIT std::atomic<bool> synch_check_invariants(false);
ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
submit_profile_data;
ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<void (*)(
const char* msg, const void* obj, int64_t wait_cycles)>
mutex_tracer;
ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
absl::base_internal::AtomicHook<void (*)(const char* msg, const void* cv)>
cond_var_tracer;
} // namespace
static inline bool EvalConditionAnnotated(const Condition* cond, Mutex* mu,
bool locking, bool trylock,
bool read_lock);
void RegisterMutexProfiler(void (*fn)(int64_t wait_cycles)) {
submit_profile_data.Store(fn);
}
void RegisterMutexTracer(void (*fn)(const char* msg, const void* obj,
int64_t wait_cycles)) {
mutex_tracer.Store(fn);
}
void RegisterCondVarTracer(void (*fn)(const char* msg, const void* cv)) {
cond_var_tracer.Store(fn);
}
namespace {
// Represents the strategy for spin and yield.
// See the comment in GetMutexGlobals() for more information.
enum DelayMode { AGGRESSIVE, GENTLE };
struct ABSL_CACHELINE_ALIGNED MutexGlobals {
absl::once_flag once;
// Note: this variable is initialized separately in Mutex::LockSlow,
// so that Mutex::Lock does not have a stack frame in optimized build.
std::atomic<int> spinloop_iterations{0};
int32_t mutex_sleep_spins[2] = {};
absl::Duration mutex_sleep_time;
};
ABSL_CONST_INIT static MutexGlobals globals;
absl::Duration MeasureTimeToYield() {
absl::Time before = absl::Now();
ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)();
return absl::Now() - before;
}
const MutexGlobals& GetMutexGlobals() {
absl::base_internal::LowLevelCallOnce(&globals.once, [&]() {
if (absl::base_internal::NumCPUs() > 1) {
// If the mode is aggressive then spin many times before yielding.
// If the mode is gentle then spin only a few times before yielding.
// Aggressive spinning is used to ensure that an Unlock() call,
// which must get the spin lock for any thread to make progress gets it
// without undue delay.
globals.mutex_sleep_spins[AGGRESSIVE] = 5000;
globals.mutex_sleep_spins[GENTLE] = 250;
globals.mutex_sleep_time = absl::Microseconds(10);
} else {
// If this a uniprocessor, only yield/sleep. Real-time threads are often
// unable to yield, so the sleep time needs to be long enough to keep
// the calling thread asleep until scheduling happens.
globals.mutex_sleep_spins[AGGRESSIVE] = 0;
globals.mutex_sleep_spins[GENTLE] = 0;
globals.mutex_sleep_time = MeasureTimeToYield() * 5;
globals.mutex_sleep_time =
std::min(globals.mutex_sleep_time, absl::Milliseconds(1));
globals.mutex_sleep_time =
std::max(globals.mutex_sleep_time, absl::Microseconds(10));
}
});
return globals;
}
} // namespace
namespace synchronization_internal {
// Returns the Mutex delay on iteration `c` depending on the given `mode`.
// The returned value should be used as `c` for the next call to `MutexDelay`.
int MutexDelay(int32_t c, int mode) {
const int32_t limit = GetMutexGlobals().mutex_sleep_spins[mode];
const absl::Duration sleep_time = GetMutexGlobals().mutex_sleep_time;
if (c < limit) {
// Spin.
c++;
} else {
SchedulingGuard::ScopedEnable enable_rescheduling;
ABSL_TSAN_MUTEX_PRE_DIVERT(nullptr, 0);
if (c == limit) {
// Yield once.
ABSL_INTERNAL_C_SYMBOL(AbslInternalMutexYield)();
c++;
} else {
// Then wait.
absl::SleepFor(sleep_time);
c = 0;
}
ABSL_TSAN_MUTEX_POST_DIVERT(nullptr, 0);
}
return c;
}
} // namespace synchronization_internal
// --------------------------Generic atomic ops
// Ensure that "(*pv & bits) == bits" by doing an atomic update of "*pv" to
// "*pv | bits" if necessary. Wait until (*pv & wait_until_clear)==0
// before making any change.
// Returns true if bits were previously unset and set by the call.
// This is used to set flags in mutex and condition variable words.
static bool AtomicSetBits(std::atomic<intptr_t>* pv, intptr_t bits,
intptr_t wait_until_clear) {
for (;;) {
intptr_t v = pv->load(std::memory_order_relaxed);
if ((v & bits) == bits) {
return false;
}
if ((v & wait_until_clear) != 0) {
continue;
}
if (pv->compare_exchange_weak(v, v | bits, std::memory_order_release,
std::memory_order_relaxed)) {
return true;
}
}
}
//------------------------------------------------------------------
// Data for doing deadlock detection.
ABSL_CONST_INIT static absl::base_internal::SpinLock deadlock_graph_mu(
absl::kConstInit, base_internal::SCHEDULE_KERNEL_ONLY);
// Graph used to detect deadlocks.
ABSL_CONST_INIT static GraphCycles* deadlock_graph
ABSL_GUARDED_BY(deadlock_graph_mu) ABSL_PT_GUARDED_BY(deadlock_graph_mu);
//------------------------------------------------------------------
// An event mechanism for debugging mutex use.
// It also allows mutexes to be given names for those who can't handle
// addresses, and instead like to give their data structures names like
// "Henry", "Fido", or "Rupert IV, King of Yondavia".
namespace { // to prevent name pollution
enum { // Mutex and CondVar events passed as "ev" to PostSynchEvent
// Mutex events
SYNCH_EV_TRYLOCK_SUCCESS,
SYNCH_EV_TRYLOCK_FAILED,
SYNCH_EV_READERTRYLOCK_SUCCESS,
SYNCH_EV_READERTRYLOCK_FAILED,
SYNCH_EV_LOCK,
SYNCH_EV_LOCK_RETURNING,
SYNCH_EV_READERLOCK,
SYNCH_EV_READERLOCK_RETURNING,
SYNCH_EV_UNLOCK,
SYNCH_EV_READERUNLOCK,
// CondVar events
SYNCH_EV_WAIT,
SYNCH_EV_WAIT_RETURNING,
SYNCH_EV_SIGNAL,
SYNCH_EV_SIGNALALL,
};
enum { // Event flags
SYNCH_F_R = 0x01, // reader event
SYNCH_F_LCK = 0x02, // PostSynchEvent called with mutex held
SYNCH_F_TRY = 0x04, // TryLock or ReaderTryLock
SYNCH_F_UNLOCK = 0x08, // Unlock or ReaderUnlock
SYNCH_F_LCK_W = SYNCH_F_LCK,
SYNCH_F_LCK_R = SYNCH_F_LCK | SYNCH_F_R,
};
} // anonymous namespace
// Properties of the events.
static const struct {
int flags;
const char* msg;
} event_properties[] = {
{SYNCH_F_LCK_W | SYNCH_F_TRY, "TryLock succeeded "},
{0, "TryLock failed "},
{SYNCH_F_LCK_R | SYNCH_F_TRY, "ReaderTryLock succeeded "},
{0, "ReaderTryLock failed "},
{0, "Lock blocking "},
{SYNCH_F_LCK_W, "Lock returning "},
{0, "ReaderLock blocking "},
{SYNCH_F_LCK_R, "ReaderLock returning "},
{SYNCH_F_LCK_W | SYNCH_F_UNLOCK, "Unlock "},
{SYNCH_F_LCK_R | SYNCH_F_UNLOCK, "ReaderUnlock "},
{0, "Wait on "},
{0, "Wait unblocked "},
{0, "Signal on "},
{0, "SignalAll on "},
};
ABSL_CONST_INIT static absl::base_internal::SpinLock synch_event_mu(
absl::kConstInit, base_internal::SCHEDULE_KERNEL_ONLY);
// Hash table size; should be prime > 2.
// Can't be too small, as it's used for deadlock detection information.
static constexpr uint32_t kNSynchEvent = 1031;
static struct SynchEvent { // this is a trivial hash table for the events
// struct is freed when refcount reaches 0
int refcount ABSL_GUARDED_BY(synch_event_mu);
// buckets have linear, 0-terminated chains
SynchEvent* next ABSL_GUARDED_BY(synch_event_mu);
// Constant after initialization
uintptr_t masked_addr; // object at this address is called "name"
// No explicit synchronization used. Instead we assume that the
// client who enables/disables invariants/logging on a Mutex does so
// while the Mutex is not being concurrently accessed by others.
void (*invariant)(void* arg); // called on each event
void* arg; // first arg to (*invariant)()
bool log; // logging turned on
// Constant after initialization
char name[1]; // actually longer---NUL-terminated string
}* synch_event[kNSynchEvent] ABSL_GUARDED_BY(synch_event_mu);
// Ensure that the object at "addr" has a SynchEvent struct associated with it,
// set "bits" in the word there (waiting until lockbit is clear before doing
// so), and return a refcounted reference that will remain valid until
// UnrefSynchEvent() is called. If a new SynchEvent is allocated,
// the string name is copied into it.
// When used with a mutex, the caller should also ensure that kMuEvent
// is set in the mutex word, and similarly for condition variables and kCVEvent.
static SynchEvent* EnsureSynchEvent(std::atomic<intptr_t>* addr,
const char* name, intptr_t bits,
intptr_t lockbit) {
uint32_t h = reinterpret_cast<uintptr_t>(addr) % kNSynchEvent;
synch_event_mu.Lock();
// When a Mutex/CondVar is destroyed, we don't remove the associated
// SynchEvent to keep destructors empty in release builds for performance
// reasons. If the current call is the first to set bits (kMuEvent/kCVEvent),
// we don't look up the existing even because (if it exists, it must be for
// the previous Mutex/CondVar that existed at the same address).
// The leaking events must not be a problem for tests, which should create
// bounded amount of events. And debug logging is not supposed to be enabled
// in production. However, if it's accidentally enabled, or briefly enabled
// for some debugging, we don't want to crash the program. Instead we drop
// all events, if we accumulated too many of them. Size of a single event
// is ~48 bytes, so 100K events is ~5 MB.
// Additionally we could delete the old event for the same address,
// but it would require a better hashmap (if we accumulate too many events,
// linked lists will grow and traversing them will be very slow).
constexpr size_t kMaxSynchEventCount = 100 << 10;
// Total number of live synch events.
static size_t synch_event_count ABSL_GUARDED_BY(synch_event_mu);
if (++synch_event_count > kMaxSynchEventCount) {
synch_event_count = 0;
ABSL_RAW_LOG(ERROR,
"Accumulated %zu Mutex debug objects. If you see this"
" in production, it may mean that the production code"
" accidentally calls "
"Mutex/CondVar::EnableDebugLog/EnableInvariantDebugging.",
kMaxSynchEventCount);
for (auto*& head : synch_event) {
for (auto* e = head; e != nullptr;) {
SynchEvent* next = e->next;
if (--(e->refcount) == 0) {
base_internal::LowLevelAlloc::Free(e);
}
e = next;
}
head = nullptr;
}
}
SynchEvent* e = nullptr;
if (!AtomicSetBits(addr, bits, lockbit)) {
for (e = synch_event[h];
e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
e = e->next) {
}
}
if (e == nullptr) { // no SynchEvent struct found; make one.
if (name == nullptr) {
name = "";
}
size_t l = strlen(name);
e = reinterpret_cast<SynchEvent*>(
base_internal::LowLevelAlloc::Alloc(sizeof(*e) + l));
e->refcount = 2; // one for return value, one for linked list
e->masked_addr = base_internal::HidePtr(addr);
e->invariant = nullptr;
e->arg = nullptr;
e->log = false;
strcpy(e->name, name); // NOLINT(runtime/printf)
e->next = synch_event[h];
synch_event[h] = e;
} else {
e->refcount++; // for return value
}
synch_event_mu.Unlock();
return e;
}
// Decrement the reference count of *e, or do nothing if e==null.
static void UnrefSynchEvent(SynchEvent* e) {
if (e != nullptr) {
synch_event_mu.Lock();
bool del = (--(e->refcount) == 0);
synch_event_mu.Unlock();
if (del) {
base_internal::LowLevelAlloc::Free(e);
}
}
}
// Return a refcounted reference to the SynchEvent of the object at address
// "addr", if any. The pointer returned is valid until the UnrefSynchEvent() is
// called.
static SynchEvent* GetSynchEvent(const void* addr) {
uint32_t h = reinterpret_cast<uintptr_t>(addr) % kNSynchEvent;
SynchEvent* e;
synch_event_mu.Lock();
for (e = synch_event[h];
e != nullptr && e->masked_addr != base_internal::HidePtr(addr);
e = e->next) {
}
if (e != nullptr) {
e->refcount++;
}
synch_event_mu.Unlock();
return e;
}
// Called when an event "ev" occurs on a Mutex of CondVar "obj"
// if event recording is on
static void PostSynchEvent(void* obj, int ev) {
SynchEvent* e = GetSynchEvent(obj);
// logging is on if event recording is on and either there's no event struct,
// or it explicitly says to log
if (e == nullptr || e->log) {
void* pcs[40];
int n = absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 1);
// A buffer with enough space for the ASCII for all the PCs, even on a
// 64-bit machine.
char buffer[ABSL_ARRAYSIZE(pcs) * 24];
int pos = snprintf(buffer, sizeof(buffer), " @");
for (int i = 0; i != n; i++) {
int b = snprintf(&buffer[pos], sizeof(buffer) - static_cast<size_t>(pos),
" %p", pcs[i]);
if (b < 0 ||
static_cast<size_t>(b) >= sizeof(buffer) - static_cast<size_t>(pos)) {
break;
}
pos += b;
}
ABSL_RAW_LOG(INFO, "%s%p %s %s", event_properties[ev].msg, obj,
(e == nullptr ? "" : e->name), buffer);
}
const int flags = event_properties[ev].flags;
if ((flags & SYNCH_F_LCK) != 0 && e != nullptr && e->invariant != nullptr) {
// Calling the invariant as is causes problems under ThreadSanitizer.
// We are currently inside of Mutex Lock/Unlock and are ignoring all
// memory accesses and synchronization. If the invariant transitively
// synchronizes something else and we ignore the synchronization, we will
// get false positive race reports later.
// Reuse EvalConditionAnnotated to properly call into user code.
struct local {
static bool pred(SynchEvent* ev) {
(*ev->invariant)(ev->arg);
return false;
}
};
Condition cond(&local::pred, e);
Mutex* mu = static_cast<Mutex*>(obj);
const bool locking = (flags & SYNCH_F_UNLOCK) == 0;
const bool trylock = (flags & SYNCH_F_TRY) != 0;
const bool read_lock = (flags & SYNCH_F_R) != 0;
EvalConditionAnnotated(&cond, mu, locking, trylock, read_lock);
}
UnrefSynchEvent(e);
}
//------------------------------------------------------------------
// The SynchWaitParams struct encapsulates the way in which a thread is waiting:
// whether it has a timeout, the condition, exclusive/shared, and whether a
// condition variable wait has an associated Mutex (as opposed to another
// type of lock). It also points to the PerThreadSynch struct of its thread.
// cv_word tells Enqueue() to enqueue on a CondVar using CondVarEnqueue().
//
// This structure is held on the stack rather than directly in
// PerThreadSynch because a thread can be waiting on multiple Mutexes if,
// while waiting on one Mutex, the implementation calls a client callback
// (such as a Condition function) that acquires another Mutex. We don't
// strictly need to allow this, but programmers become confused if we do not
// allow them to use functions such a LOG() within Condition functions. The
// PerThreadSynch struct points at the most recent SynchWaitParams struct when
// the thread is on a Mutex's waiter queue.
struct SynchWaitParams {
SynchWaitParams(Mutex::MuHow how_arg, const Condition* cond_arg,
KernelTimeout timeout_arg, Mutex* cvmu_arg,
PerThreadSynch* thread_arg,
std::atomic<intptr_t>* cv_word_arg)
: how(how_arg),
cond(cond_arg),
timeout(timeout_arg),
cvmu(cvmu_arg),
thread(thread_arg),
cv_word(cv_word_arg),
contention_start_cycles(CycleClock::Now()),
should_submit_contention_data(false) {}
const Mutex::MuHow how; // How this thread needs to wait.
const Condition* cond; // The condition that this thread is waiting for.
// In Mutex, this field is set to zero if a timeout
// expires.
KernelTimeout timeout; // timeout expiry---absolute time
// In Mutex, this field is set to zero if a timeout
// expires.
Mutex* const cvmu; // used for transfer from cond var to mutex
PerThreadSynch* const thread; // thread that is waiting
// If not null, thread should be enqueued on the CondVar whose state
// word is cv_word instead of queueing normally on the Mutex.
std::atomic<intptr_t>* cv_word;
int64_t contention_start_cycles; // Time (in cycles) when this thread started
// to contend for the mutex.
bool should_submit_contention_data;
};
struct SynchLocksHeld {
int n; // number of valid entries in locks[]
bool overflow; // true iff we overflowed the array at some point
struct {
Mutex* mu; // lock acquired
int32_t count; // times acquired
GraphId id; // deadlock_graph id of acquired lock
} locks[40];
// If a thread overfills the array during deadlock detection, we
// continue, discarding information as needed. If no overflow has
// taken place, we can provide more error checking, such as
// detecting when a thread releases a lock it does not hold.
};
// A sentinel value in lists that is not 0.
// A 0 value is used to mean "not on a list".
static PerThreadSynch* const kPerThreadSynchNull =
reinterpret_cast<PerThreadSynch*>(1);
static SynchLocksHeld* LocksHeldAlloc() {
SynchLocksHeld* ret = reinterpret_cast<SynchLocksHeld*>(
base_internal::LowLevelAlloc::Alloc(sizeof(SynchLocksHeld)));
ret->n = 0;
ret->overflow = false;
return ret;
}
// Return the PerThreadSynch-struct for this thread.
static PerThreadSynch* Synch_GetPerThread() {
ThreadIdentity* identity = GetOrCreateCurrentThreadIdentity();
return &identity->per_thread_synch;
}
static PerThreadSynch* Synch_GetPerThreadAnnotated(Mutex* mu) {
if (mu) {
ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
}
PerThreadSynch* w = Synch_GetPerThread();
if (mu) {
ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
}
return w;
}
static SynchLocksHeld* Synch_GetAllLocks() {
PerThreadSynch* s = Synch_GetPerThread();
if (s->all_locks == nullptr) {
s->all_locks = LocksHeldAlloc(); // Freed by ReclaimThreadIdentity.
}
return s->all_locks;
}
// Post on "w"'s associated PerThreadSem.
void Mutex::IncrementSynchSem(Mutex* mu, PerThreadSynch* w) {
static_cast<void>(mu); // Prevent unused param warning in non-TSAN builds.
ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
// We miss synchronization around passing PerThreadSynch between threads
// since it happens inside of the Mutex code, so we need to ignore all
// accesses to the object.
ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
PerThreadSem::Post(w->thread_identity());
ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
}
// Wait on "w"'s associated PerThreadSem; returns false if timeout expired.
bool Mutex::DecrementSynchSem(Mutex* mu, PerThreadSynch* w, KernelTimeout t) {
static_cast<void>(mu); // Prevent unused param warning in non-TSAN builds.
ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
assert(w == Synch_GetPerThread());
static_cast<void>(w);
bool res = PerThreadSem::Wait(t);
ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
return res;
}
// We're in a fatal signal handler that hopes to use Mutex and to get
// lucky by not deadlocking. We try to improve its chances of success
// by effectively disabling some of the consistency checks. This will
// prevent certain ABSL_RAW_CHECK() statements from being triggered when
// re-rentry is detected. The ABSL_RAW_CHECK() statements are those in the
// Mutex code checking that the "waitp" field has not been reused.
void Mutex::InternalAttemptToUseMutexInFatalSignalHandler() {
// Fix the per-thread state only if it exists.
ThreadIdentity* identity = CurrentThreadIdentityIfPresent();
if (identity != nullptr) {
identity->per_thread_synch.suppress_fatal_errors = true;
}
// Don't do deadlock detection when we are already failing.
synch_deadlock_detection.store(OnDeadlockCycle::kIgnore,
std::memory_order_release);
}
// --------------------------Mutexes
// In the layout below, the msb of the bottom byte is currently unused. Also,
// the following constraints were considered in choosing the layout:
// o Both the debug allocator's "uninitialized" and "freed" patterns (0xab and
// 0xcd) are illegal: reader and writer lock both held.
// o kMuWriter and kMuEvent should exceed kMuDesig and kMuWait, to enable the
// bit-twiddling trick in Mutex::Unlock().
// o kMuWriter / kMuReader == kMuWrWait / kMuWait,
// to enable the bit-twiddling trick in CheckForMutexCorruption().
static const intptr_t kMuReader = 0x0001L; // a reader holds the lock
// There's a designated waker.
// INVARIANT1: there's a thread that was blocked on the mutex, is
// no longer, yet has not yet acquired the mutex. If there's a
// designated waker, all threads can avoid taking the slow path in
// unlock because the designated waker will subsequently acquire
// the lock and wake someone. To maintain INVARIANT1 the bit is
// set when a thread is unblocked(INV1a), and threads that were
// unblocked reset the bit when they either acquire or re-block (INV1b).
static const intptr_t kMuDesig = 0x0002L;
static const intptr_t kMuWait = 0x0004L; // threads are waiting
static const intptr_t kMuWriter = 0x0008L; // a writer holds the lock
static const intptr_t kMuEvent = 0x0010L; // record this mutex's events
// Runnable writer is waiting for a reader.
// If set, new readers will not lock the mutex to avoid writer starvation.
// Note: if a reader has higher priority than the writer, it will still lock
// the mutex ahead of the waiting writer, but in a very inefficient manner:
// the reader will first queue itself and block, but then the last unlocking
// reader will wake it.
static const intptr_t kMuWrWait = 0x0020L;
static const intptr_t kMuSpin = 0x0040L; // spinlock protects wait list
static const intptr_t kMuLow = 0x00ffL; // mask all mutex bits
static const intptr_t kMuHigh = ~kMuLow; // mask pointer/reader count
// Hack to make constant values available to gdb pretty printer
enum {
kGdbMuSpin = kMuSpin,
kGdbMuEvent = kMuEvent,
kGdbMuWait = kMuWait,
kGdbMuWriter = kMuWriter,
kGdbMuDesig = kMuDesig,
kGdbMuWrWait = kMuWrWait,
kGdbMuReader = kMuReader,
kGdbMuLow = kMuLow,
};
// kMuWrWait implies kMuWait.
// kMuReader and kMuWriter are mutually exclusive.
// If kMuReader is zero, there are no readers.
// Otherwise, if kMuWait is zero, the high order bits contain a count of the
// number of readers. Otherwise, the reader count is held in
// PerThreadSynch::readers of the most recently queued waiter, again in the
// bits above kMuLow.
static const intptr_t kMuOne = 0x0100; // a count of one reader
// flags passed to Enqueue and LockSlow{,WithTimeout,Loop}
static const int kMuHasBlocked = 0x01; // already blocked (MUST == 1)
static const int kMuIsCond = 0x02; // conditional waiter (CV or Condition)
static const int kMuIsFer = 0x04; // wait morphing from a CondVar
static_assert(PerThreadSynch::kAlignment > kMuLow,
"PerThreadSynch::kAlignment must be greater than kMuLow");
// This struct contains various bitmasks to be used in
// acquiring and releasing a mutex in a particular mode.
struct MuHowS {
// if all the bits in fast_need_zero are zero, the lock can be acquired by
// adding fast_add and oring fast_or. The bit kMuDesig should be reset iff
// this is the designated waker.
intptr_t fast_need_zero;
intptr_t fast_or;
intptr_t fast_add;
intptr_t slow_need_zero; // fast_need_zero with events (e.g. logging)
intptr_t slow_inc_need_zero; // if all the bits in slow_inc_need_zero are
// zero a reader can acquire a read share by
// setting the reader bit and incrementing
// the reader count (in last waiter since
// we're now slow-path). kMuWrWait be may
// be ignored if we already waited once.
};
static const MuHowS kSharedS = {
// shared or read lock
kMuWriter | kMuWait | kMuEvent, // fast_need_zero
kMuReader, // fast_or
kMuOne, // fast_add
kMuWriter | kMuWait, // slow_need_zero
kMuSpin | kMuWriter | kMuWrWait, // slow_inc_need_zero
};
static const MuHowS kExclusiveS = {
// exclusive or write lock
kMuWriter | kMuReader | kMuEvent, // fast_need_zero
kMuWriter, // fast_or
0, // fast_add
kMuWriter | kMuReader, // slow_need_zero
~static_cast<intptr_t>(0), // slow_inc_need_zero
};
static const Mutex::MuHow kShared = &kSharedS; // shared lock
static const Mutex::MuHow kExclusive = &kExclusiveS; // exclusive lock
#ifdef NDEBUG
static constexpr bool kDebugMode = false;
#else
static constexpr bool kDebugMode = true;
#endif
#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
static unsigned TsanFlags(Mutex::MuHow how) {
return how == kShared ? __tsan_mutex_read_lock : 0;
}
#endif
#if defined(__APPLE__) || defined(ABSL_BUILD_DLL)
// When building a dll symbol export lists may reference the destructor
// and want it to be an exported symbol rather than an inline function.
// Some apple builds also do dynamic library build but don't say it explicitly.
Mutex::~Mutex() { Dtor(); }
#endif
#if !defined(NDEBUG) || defined(ABSL_HAVE_THREAD_SANITIZER)
void Mutex::Dtor() {
if (kDebugMode) {
this->ForgetDeadlockInfo();
}
ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static);
}
#endif
void Mutex::EnableDebugLog(const char* name) {
// Need to disable writes here and in EnableInvariantDebugging to prevent
// false race reports on SynchEvent objects. TSan ignores synchronization
// on synch_event_mu in Lock/Unlock/etc methods due to mutex annotations,
// but it sees few accesses to SynchEvent in EvalConditionAnnotated.
// If we don't ignore accesses here, it can result in false races
// between EvalConditionAnnotated and SynchEvent reuse in EnsureSynchEvent.
ABSL_ANNOTATE_IGNORE_WRITES_BEGIN();
SynchEvent* e = EnsureSynchEvent(&this->mu_, name, kMuEvent, kMuSpin);
e->log = true;
UnrefSynchEvent(e);
// This prevents "error: undefined symbol: absl::Mutex::~Mutex()"
// in a release build (NDEBUG defined) when a test does "#undef NDEBUG"
// to use assert macro. In such case, the test does not get the dtor
// definition because it's supposed to be outline when NDEBUG is not defined,
// and this source file does not define one either because NDEBUG is defined.
// Since it's not possible to take address of a destructor, we move the
// actual destructor code into the separate Dtor function and force the
// compiler to emit this function even if it's inline by taking its address.
ABSL_ATTRIBUTE_UNUSED volatile auto dtor = &Mutex::Dtor;
ABSL_ANNOTATE_IGNORE_WRITES_END();
}
void EnableMutexInvariantDebugging(bool enabled) {
synch_check_invariants.store(enabled, std::memory_order_release);
}
void Mutex::EnableInvariantDebugging(void (*invariant)(void*), void* arg) {
ABSL_ANNOTATE_IGNORE_WRITES_BEGIN();
if (synch_check_invariants.load(std::memory_order_acquire) &&
invariant != nullptr) {
SynchEvent* e = EnsureSynchEvent(&this->mu_, nullptr, kMuEvent, kMuSpin);
e->invariant = invariant;
e->arg = arg;
UnrefSynchEvent(e);
}
ABSL_ANNOTATE_IGNORE_WRITES_END();
}
void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode) {
synch_deadlock_detection.store(mode, std::memory_order_release);
}
// Return true iff threads x and y are part of the same equivalence
// class of waiters. An equivalence class is defined as the set of
// waiters with the same condition, type of lock, and thread priority.
//
// Requires that x and y be waiting on the same Mutex queue.
static bool MuEquivalentWaiter(PerThreadSynch* x, PerThreadSynch* y) {
return x->waitp->how == y->waitp->how && x->priority == y->priority &&
Condition::GuaranteedEqual(x->waitp->cond, y->waitp->cond);
}
// Given the contents of a mutex word containing a PerThreadSynch pointer,
// return the pointer.
static inline PerThreadSynch* GetPerThreadSynch(intptr_t v) {
return reinterpret_cast<PerThreadSynch*>(v & kMuHigh);
}
// The next several routines maintain the per-thread next and skip fields
// used in the Mutex waiter queue.
// The queue is a circular singly-linked list, of which the "head" is the
// last element, and head->next if the first element.
// The skip field has the invariant:
// For thread x, x->skip is one of:
// - invalid (iff x is not in a Mutex wait queue),
// - null, or
// - a pointer to a distinct thread waiting later in the same Mutex queue
// such that all threads in [x, x->skip] have the same condition, priority
// and lock type (MuEquivalentWaiter() is true for all pairs in [x,
// x->skip]).
// In addition, if x->skip is valid, (x->may_skip || x->skip == null)
//
// By the spec of MuEquivalentWaiter(), it is not necessary when removing the
// first runnable thread y from the front a Mutex queue to adjust the skip
// field of another thread x because if x->skip==y, x->skip must (have) become
// invalid before y is removed. The function TryRemove can remove a specified
// thread from an arbitrary position in the queue whether runnable or not, so
// it fixes up skip fields that would otherwise be left dangling.
// The statement
// if (x->may_skip && MuEquivalentWaiter(x, x->next)) { x->skip = x->next; }
// maintains the invariant provided x is not the last waiter in a Mutex queue
// The statement
// if (x->skip != null) { x->skip = x->skip->skip; }
// maintains the invariant.
// Returns the last thread y in a mutex waiter queue such that all threads in
// [x, y] inclusive share the same condition. Sets skip fields of some threads
// in that range to optimize future evaluation of Skip() on x values in
// the range. Requires thread x is in a mutex waiter queue.
// The locking is unusual. Skip() is called under these conditions:
// - spinlock is held in call from Enqueue(), with maybe_unlocking == false
// - Mutex is held in call from UnlockSlow() by last unlocker, with
// maybe_unlocking == true
// - both Mutex and spinlock are held in call from DequeueAllWakeable() (from
// UnlockSlow()) and TryRemove()
// These cases are mutually exclusive, so Skip() never runs concurrently
// with itself on the same Mutex. The skip chain is used in these other places
// that cannot occur concurrently:
// - FixSkip() (from TryRemove()) - spinlock and Mutex are held)
// - Dequeue() (with spinlock and Mutex held)
// - UnlockSlow() (with spinlock and Mutex held)
// A more complex case is Enqueue()
// - Enqueue() (with spinlock held and maybe_unlocking == false)
// This is the first case in which Skip is called, above.
// - Enqueue() (without spinlock held; but queue is empty and being freshly
// formed)
// - Enqueue() (with spinlock held and maybe_unlocking == true)
// The first case has mutual exclusion, and the second isolation through
// working on an otherwise unreachable data structure.
// In the last case, Enqueue() is required to change no skip/next pointers
// except those in the added node and the former "head" node. This implies
// that the new node is added after head, and so must be the new head or the
// new front of the queue.
static PerThreadSynch* Skip(PerThreadSynch* x) {
PerThreadSynch* x0 = nullptr;
PerThreadSynch* x1 = x;
PerThreadSynch* x2 = x->skip;
if (x2 != nullptr) {
// Each iteration attempts to advance sequence (x0,x1,x2) to next sequence
// such that x1 == x0->skip && x2 == x1->skip
while ((x0 = x1, x1 = x2, x2 = x2->skip) != nullptr) {
x0->skip = x2; // short-circuit skip from x0 to x2
}
x->skip = x1; // short-circuit skip from x to result
}
return x1;
}
// "ancestor" appears before "to_be_removed" in the same Mutex waiter queue.
// The latter is going to be removed out of order, because of a timeout.
// Check whether "ancestor" has a skip field pointing to "to_be_removed",
// and fix it if it does.
static void FixSkip(PerThreadSynch* ancestor, PerThreadSynch* to_be_removed) {
if (ancestor->skip == to_be_removed) { // ancestor->skip left dangling
if (to_be_removed->skip != nullptr) {
ancestor->skip = to_be_removed->skip; // can skip past to_be_removed
} else if (ancestor->next != to_be_removed) { // they are not adjacent
ancestor->skip = ancestor->next; // can skip one past ancestor
} else {
ancestor->skip = nullptr; // can't skip at all
}
}
}
static void CondVarEnqueue(SynchWaitParams* waitp);
// Enqueue thread "waitp->thread" on a waiter queue.
// Called with mutex spinlock held if head != nullptr
// If head==nullptr and waitp->cv_word==nullptr, then Enqueue() is
// idempotent; it alters no state associated with the existing (empty)
// queue.
//
// If waitp->cv_word == nullptr, queue the thread at either the front or
// the end (according to its priority) of the circular mutex waiter queue whose
// head is "head", and return the new head. mu is the previous mutex state,
// which contains the reader count (perhaps adjusted for the operation in
// progress) if the list was empty and a read lock held, and the holder hint if
// the list was empty and a write lock held. (flags & kMuIsCond) indicates
// whether this thread was transferred from a CondVar or is waiting for a
// non-trivial condition. In this case, Enqueue() never returns nullptr
//
// If waitp->cv_word != nullptr, CondVarEnqueue() is called, and "head" is
// returned. This mechanism is used by CondVar to queue a thread on the
// condition variable queue instead of the mutex queue in implementing Wait().
// In this case, Enqueue() can return nullptr (if head==nullptr).
static PerThreadSynch* Enqueue(PerThreadSynch* head, SynchWaitParams* waitp,
intptr_t mu, int flags) {
// If we have been given a cv_word, call CondVarEnqueue() and return
// the previous head of the Mutex waiter queue.
if (waitp->cv_word != nullptr) {
CondVarEnqueue(waitp);
return head;
}
PerThreadSynch* s = waitp->thread;
ABSL_RAW_CHECK(
s->waitp == nullptr || // normal case
s->waitp == waitp || // Fer()---transfer from condition variable
s->suppress_fatal_errors,
"detected illegal recursion into Mutex code");
s->waitp = waitp;
s->skip = nullptr; // maintain skip invariant (see above)
s->may_skip = true; // always true on entering queue
s->wake = false; // not being woken
s->cond_waiter = ((flags & kMuIsCond) != 0);
#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
if ((flags & kMuIsFer) == 0) {
assert(s == Synch_GetPerThread());
int64_t now_cycles = CycleClock::Now();
if (s->next_priority_read_cycles < now_cycles) {
// Every so often, update our idea of the thread's priority.
// pthread_getschedparam() is 5% of the block/wakeup time;
// CycleClock::Now() is 0.5%.
int policy;
struct sched_param param;
const int err = pthread_getschedparam(pthread_self(), &policy, &param);
if (err != 0) {
ABSL_RAW_LOG(ERROR, "pthread_getschedparam failed: %d", err);
} else {
s->priority = param.sched_priority;
s->next_priority_read_cycles =
now_cycles + static_cast<int64_t>(CycleClock::Frequency());
}
}
}
#endif
if (head == nullptr) { // s is the only waiter
s->next = s; // it's the only entry in the cycle
s->readers = mu; // reader count is from mu word
s->maybe_unlocking = false; // no one is searching an empty list
head = s; // s is new head
} else {
PerThreadSynch* enqueue_after = nullptr; // we'll put s after this element
#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
if (s->priority > head->priority) { // s's priority is above head's
// try to put s in priority-fifo order, or failing that at the front.
if (!head->maybe_unlocking) {
// No unlocker can be scanning the queue, so we can insert into the
// middle of the queue.
//
// Within a skip chain, all waiters have the same priority, so we can
// skip forward through the chains until we find one with a lower
// priority than the waiter to be enqueued.
PerThreadSynch* advance_to = head; // next value of enqueue_after
do {
enqueue_after = advance_to;
// (side-effect: optimizes skip chain)
advance_to = Skip(enqueue_after->next);
} while (s->priority <= advance_to->priority);
// termination guaranteed because s->priority > head->priority
// and head is the end of a skip chain
} else if (waitp->how == kExclusive && waitp->cond == nullptr) {
// An unlocker could be scanning the queue, but we know it will recheck
// the queue front for writers that have no condition, which is what s
// is, so an insert at front is safe.
enqueue_after = head; // add after head, at front
}
}
#endif
if (enqueue_after != nullptr) {
s->next = enqueue_after->next;
enqueue_after->next = s;
// enqueue_after can be: head, Skip(...), or cur.
// The first two imply enqueue_after->skip == nullptr, and
// the last is used only if MuEquivalentWaiter(s, cur).
// We require this because clearing enqueue_after->skip
// is impossible; enqueue_after's predecessors might also
// incorrectly skip over s if we were to allow other
// insertion points.
ABSL_RAW_CHECK(enqueue_after->skip == nullptr ||
MuEquivalentWaiter(enqueue_after, s),
"Mutex Enqueue failure");
if (enqueue_after != head && enqueue_after->may_skip &&
MuEquivalentWaiter(enqueue_after, enqueue_after->next)) {
// enqueue_after can skip to its new successor, s
enqueue_after->skip = enqueue_after->next;
}
if (MuEquivalentWaiter(s, s->next)) { // s->may_skip is known to be true
s->skip = s->next; // s may skip to its successor
}
} else if ((flags & kMuHasBlocked) &&
(s->priority >= head->next->priority) &&
(!head->maybe_unlocking ||
(waitp->how == kExclusive &&
Condition::GuaranteedEqual(waitp->cond, nullptr)))) {
// This thread has already waited, then was woken, then failed to acquire
// the mutex and now tries to requeue. Try to requeue it at head,
// otherwise it can suffer bad latency (wait whole queue several times).
// However, we need to be conservative. First, we need to ensure that we
// respect priorities. Then, we need to be careful to not break wait
// queue invariants: we require either that unlocker is not scanning
// the queue or that the current thread is a writer with no condition
// (unlocker will recheck the queue for such waiters).
s->next = head->next;
head->next = s;
if (MuEquivalentWaiter(s, s->next)) { // s->may_skip is known to be true
s->skip = s->next; // s may skip to its successor
}
} else { // enqueue not done any other way, so
// we're inserting s at the back
// s will become new head; copy data from head into it
s->next = head->next; // add s after head
head->next = s;
s->readers = head->readers; // reader count is from previous head
s->maybe_unlocking = head->maybe_unlocking; // same for unlock hint
if (head->may_skip && MuEquivalentWaiter(head, s)) {
// head now has successor; may skip
head->skip = s;
}
head = s; // s is new head
}
}
s->state.store(PerThreadSynch::kQueued, std::memory_order_relaxed);
return head;
}
// Dequeue the successor pw->next of thread pw from the Mutex waiter queue
// whose last element is head. The new head element is returned, or null
// if the list is made empty.
// Dequeue is called with both spinlock and Mutex held.
static PerThreadSynch* Dequeue(PerThreadSynch* head, PerThreadSynch* pw) {
PerThreadSynch* w = pw->next;
pw->next = w->next; // snip w out of list
if (head == w) { // we removed the head
head = (pw == w) ? nullptr : pw; // either emptied list, or pw is new head
} else if (pw != head && MuEquivalentWaiter(pw, pw->next)) {
// pw can skip to its new successor
if (pw->next->skip !=
nullptr) { // either skip to its successors skip target
pw->skip = pw->next->skip;
} else { // or to pw's successor
pw->skip = pw->next;
}
}
return head;
}
// Traverse the elements [ pw->next, h] of the circular list whose last element
// is head.
// Remove all elements with wake==true and place them in the
// singly-linked list wake_list in the order found. Assumes that
// there is only one such element if the element has how == kExclusive.
// Return the new head.
static PerThreadSynch* DequeueAllWakeable(PerThreadSynch* head,
PerThreadSynch* pw,
PerThreadSynch** wake_tail) {
PerThreadSynch* orig_h = head;
PerThreadSynch* w = pw->next;
bool skipped = false;
do {
if (w->wake) { // remove this element
ABSL_RAW_CHECK(pw->skip == nullptr, "bad skip in DequeueAllWakeable");
// we're removing pw's successor so either pw->skip is zero or we should
// already have removed pw since if pw->skip!=null, pw has the same
// condition as w.
head = Dequeue(head, pw);
w->next = *wake_tail; // keep list terminated
*wake_tail = w; // add w to wake_list;
wake_tail = &w->next; // next addition to end
if (w->waitp->how == kExclusive) { // wake at most 1 writer
break;
}
} else { // not waking this one; skip
pw = Skip(w); // skip as much as possible
skipped = true;
}
w = pw->next;
// We want to stop processing after we've considered the original head,
// orig_h. We can't test for w==orig_h in the loop because w may skip over
// it; we are guaranteed only that w's predecessor will not skip over
// orig_h. When we've considered orig_h, either we've processed it and
// removed it (so orig_h != head), or we considered it and skipped it (so
// skipped==true && pw == head because skipping from head always skips by
// just one, leaving pw pointing at head). So we want to
// continue the loop with the negation of that expression.
} while (orig_h == head && (pw != head || !skipped));
return head;
}
// Try to remove thread s from the list of waiters on this mutex.
// Does nothing if s is not on the waiter list.
void Mutex::TryRemove(PerThreadSynch* s) {
SchedulingGuard::ScopedDisable disable_rescheduling;
intptr_t v = mu_.load(std::memory_order_relaxed);
// acquire spinlock & lock
if ((v & (kMuWait | kMuSpin | kMuWriter | kMuReader)) == kMuWait &&
mu_.compare_exchange_strong(v, v | kMuSpin | kMuWriter,
std::memory_order_acquire,
std::memory_order_relaxed)) {
PerThreadSynch* h = GetPerThreadSynch(v);
if (h != nullptr) {
PerThreadSynch* pw = h; // pw is w's predecessor
PerThreadSynch* w;
if ((w = pw->next) != s) { // search for thread,
do { // processing at least one element
// If the current element isn't equivalent to the waiter to be
// removed, we can skip the entire chain.
if (!MuEquivalentWaiter(s, w)) {
pw = Skip(w); // so skip all that won't match
// we don't have to worry about dangling skip fields
// in the threads we skipped; none can point to s
// because they are in a different equivalence class.
} else { // seeking same condition
FixSkip(w, s); // fix up any skip pointer from w to s
pw = w;
}
// don't search further if we found the thread, or we're about to
// process the first thread again.
} while ((w = pw->next) != s && pw != h);
}
if (w == s) { // found thread; remove it
// pw->skip may be non-zero here; the loop above ensured that
// no ancestor of s can skip to s, so removal is safe anyway.
h = Dequeue(h, pw);
s->next = nullptr;
s->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
}
}
intptr_t nv;
do { // release spinlock and lock
v = mu_.load(std::memory_order_relaxed);
nv = v & (kMuDesig | kMuEvent);
if (h != nullptr) {
nv |= kMuWait | reinterpret_cast<intptr_t>(h);
h->readers = 0; // we hold writer lock
h->maybe_unlocking = false; // finished unlocking
}
} while (!mu_.compare_exchange_weak(v, nv, std::memory_order_release,
std::memory_order_relaxed));
}
}
// Wait until thread "s", which must be the current thread, is removed from the
// this mutex's waiter queue. If "s->waitp->timeout" has a timeout, wake up
// if the wait extends past the absolute time specified, even if "s" is still
// on the mutex queue. In this case, remove "s" from the queue and return
// true, otherwise return false.
void Mutex::Block(PerThreadSynch* s) {
while (s->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) {
if (!DecrementSynchSem(this, s, s->waitp->timeout)) {
// After a timeout, we go into a spin loop until we remove ourselves
// from the queue, or someone else removes us. We can't be sure to be
// able to remove ourselves in a single lock acquisition because this
// mutex may be held, and the holder has the right to read the centre
// of the waiter queue without holding the spinlock.
this->TryRemove(s);
int c = 0;
while (s->next != nullptr) {
c = synchronization_internal::MutexDelay(c, GENTLE);
this->TryRemove(s);
}
if (kDebugMode) {
// This ensures that we test the case that TryRemove() is called when s
// is not on the queue.
this->TryRemove(s);
}
s->waitp->timeout = KernelTimeout::Never(); // timeout is satisfied
s->waitp->cond = nullptr; // condition no longer relevant for wakeups
}
}
ABSL_RAW_CHECK(s->waitp != nullptr || s->suppress_fatal_errors,
"detected illegal recursion in Mutex code");
s->waitp = nullptr;
}
// Wake thread w, and return the next thread in the list.
PerThreadSynch* Mutex::Wakeup(PerThreadSynch* w) {
PerThreadSynch* next = w->next;
w->next = nullptr;
w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
IncrementSynchSem(this, w);
return next;
}
static GraphId GetGraphIdLocked(Mutex* mu)
ABSL_EXCLUSIVE_LOCKS_REQUIRED(deadlock_graph_mu) {
if (!deadlock_graph) { // (re)create the deadlock graph.
deadlock_graph =
new (base_internal::LowLevelAlloc::Alloc(sizeof(*deadlock_graph)))
GraphCycles;
}
return deadlock_graph->GetId(mu);
}
static GraphId GetGraphId(Mutex* mu) ABSL_LOCKS_EXCLUDED(deadlock_graph_mu) {
deadlock_graph_mu.Lock();
GraphId id = GetGraphIdLocked(mu);
deadlock_graph_mu.Unlock();
return id;
}
// Record a lock acquisition. This is used in debug mode for deadlock
// detection. The held_locks pointer points to the relevant data
// structure for each case.
static void LockEnter(Mutex* mu, GraphId id, SynchLocksHeld* held_locks) {
int n = held_locks->n;
int i = 0;
while (i != n && held_locks->locks[i].id != id) {
i++;
}
if (i == n) {
if (n == ABSL_ARRAYSIZE(held_locks->locks)) {
held_locks->overflow = true; // lost some data
} else { // we have room for lock
held_locks->locks[i].mu = mu;
held_locks->locks[i].count = 1;
held_locks->locks[i].id = id;
held_locks->n = n + 1;
}
} else {
held_locks->locks[i].count++;
}
}
// Record a lock release. Each call to LockEnter(mu, id, x) should be
// eventually followed by a call to LockLeave(mu, id, x) by the same thread.
// It does not process the event if is not needed when deadlock detection is
// disabled.
static void LockLeave(Mutex* mu, GraphId id, SynchLocksHeld* held_locks) {
int n = held_locks->n;
int i = 0;
while (i != n && held_locks->locks[i].id != id) {
i++;
}
if (i == n) {
if (!held_locks->overflow) {
// The deadlock id may have been reassigned after ForgetDeadlockInfo,
// but in that case mu should still be present.
i = 0;
while (i != n && held_locks->locks[i].mu != mu) {
i++;
}
if (i == n) { // mu missing means releasing unheld lock
SynchEvent* mu_events = GetSynchEvent(mu);
ABSL_RAW_LOG(FATAL,
"thread releasing lock it does not hold: %p %s; "
,
static_cast<void*>(mu),
mu_events == nullptr ? "" : mu_events->name);
}
}
} else if (held_locks->locks[i].count == 1) {
held_locks->n = n - 1;
held_locks->locks[i] = held_locks->locks[n - 1];
held_locks->locks[n - 1].id = InvalidGraphId();
held_locks->locks[n - 1].mu =
nullptr; // clear mu to please the leak detector.
} else {
assert(held_locks->locks[i].count > 0);
held_locks->locks[i].count--;
}
}
// Call LockEnter() if in debug mode and deadlock detection is enabled.
static inline void DebugOnlyLockEnter(Mutex* mu) {
if (kDebugMode) {
if (synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
LockEnter(mu, GetGraphId(mu), Synch_GetAllLocks());
}
}
}
// Call LockEnter() if in debug mode and deadlock detection is enabled.
static inline void DebugOnlyLockEnter(Mutex* mu, GraphId id) {
if (kDebugMode) {
if (synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
LockEnter(mu, id, Synch_GetAllLocks());
}
}
}
// Call LockLeave() if in debug mode and deadlock detection is enabled.
static inline void DebugOnlyLockLeave(Mutex* mu) {
if (kDebugMode) {
if (synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
LockLeave(mu, GetGraphId(mu), Synch_GetAllLocks());
}
}
}
static char* StackString(void** pcs, int n, char* buf, int maxlen,
bool symbolize) {
static constexpr int kSymLen = 200;
char sym[kSymLen];
int len = 0;
for (int i = 0; i != n; i++) {
if (len >= maxlen)
return buf;
size_t count = static_cast<size_t>(maxlen - len);
if (symbolize) {
if (!absl::Symbolize(pcs[i], sym, kSymLen)) {
sym[0] = '\0';
}
snprintf(buf + len, count, "%s\t@ %p %s\n", (i == 0 ? "\n" : ""), pcs[i],
sym);
} else {
snprintf(buf + len, count, " %p", pcs[i]);
}
len += strlen(&buf[len]);
}
return buf;
}
static char* CurrentStackString(char* buf, int maxlen, bool symbolize) {
void* pcs[40];
return StackString(pcs, absl::GetStackTrace(pcs, ABSL_ARRAYSIZE(pcs), 2), buf,
maxlen, symbolize);
}
namespace {
enum {
kMaxDeadlockPathLen = 10
}; // maximum length of a deadlock cycle;
// a path this long would be remarkable
// Buffers required to report a deadlock.
// We do not allocate them on stack to avoid large stack frame.
struct DeadlockReportBuffers {
char buf[6100];
GraphId path[kMaxDeadlockPathLen];
};
struct ScopedDeadlockReportBuffers {
ScopedDeadlockReportBuffers() {
b = reinterpret_cast<DeadlockReportBuffers*>(
base_internal::LowLevelAlloc::Alloc(sizeof(*b)));
}
~ScopedDeadlockReportBuffers() { base_internal::LowLevelAlloc::Free(b); }
DeadlockReportBuffers* b;
};
// Helper to pass to GraphCycles::UpdateStackTrace.
int GetStack(void** stack, int max_depth) {
return absl::GetStackTrace(stack, max_depth, 3);
}
} // anonymous namespace
// Called in debug mode when a thread is about to acquire a lock in a way that
// may block.
static GraphId DeadlockCheck(Mutex* mu) {
if (synch_deadlock_detection.load(std::memory_order_acquire) ==
OnDeadlockCycle::kIgnore) {
return InvalidGraphId();
}
SynchLocksHeld* all_locks = Synch_GetAllLocks();
absl::base_internal::SpinLockHolder lock(&deadlock_graph_mu);
const GraphId mu_id = GetGraphIdLocked(mu);
if (all_locks->n == 0) {
// There are no other locks held. Return now so that we don't need to
// call GetSynchEvent(). This way we do not record the stack trace
// for this Mutex. It's ok, since if this Mutex is involved in a deadlock,
// it can't always be the first lock acquired by a thread.
return mu_id;
}
// We prefer to keep stack traces that show a thread holding and acquiring
// as many locks as possible. This increases the chances that a given edge
// in the acquires-before graph will be represented in the stack traces
// recorded for the locks.
deadlock_graph->UpdateStackTrace(mu_id, all_locks->n + 1, GetStack);
// For each other mutex already held by this thread:
for (int i = 0; i != all_locks->n; i++) {
const GraphId other_node_id = all_locks->locks[i].id;
const Mutex* other =
static_cast<const Mutex*>(deadlock_graph->Ptr(other_node_id));
if (other == nullptr) {
// Ignore stale lock
continue;
}
// Add the acquired-before edge to the graph.
if (!deadlock_graph->InsertEdge(other_node_id, mu_id)) {
ScopedDeadlockReportBuffers scoped_buffers;
DeadlockReportBuffers* b = scoped_buffers.b;
static int number_of_reported_deadlocks = 0;
number_of_reported_deadlocks++;
// Symbolize only 2 first deadlock report to avoid huge slowdowns.
bool symbolize = number_of_reported_deadlocks <= 2;
ABSL_RAW_LOG(ERROR, "Potential Mutex deadlock: %s",
CurrentStackString(b->buf, sizeof (b->buf), symbolize));
size_t len = 0;
for (int j = 0; j != all_locks->n; j++) {
void* pr = deadlock_graph->Ptr(all_locks->locks[j].id);
if (pr != nullptr) {
snprintf(b->buf + len, sizeof(b->buf) - len, " %p", pr);
len += strlen(&b->buf[len]);
}
}
ABSL_RAW_LOG(ERROR,
"Acquiring absl::Mutex %p while holding %s; a cycle in the "
"historical lock ordering graph has been observed",
static_cast<void*>(mu), b->buf);
ABSL_RAW_LOG(ERROR, "Cycle: ");
int path_len = deadlock_graph->FindPath(mu_id, other_node_id,
ABSL_ARRAYSIZE(b->path), b->path);
for (int j = 0; j != path_len && j != ABSL_ARRAYSIZE(b->path); j++) {
GraphId id = b->path[j];
Mutex* path_mu = static_cast<Mutex*>(deadlock_graph->Ptr(id));
if (path_mu == nullptr) continue;
void** stack;
int depth = deadlock_graph->GetStackTrace(id, &stack);
snprintf(b->buf, sizeof(b->buf),
"mutex@%p stack: ", static_cast<void*>(path_mu));
StackString(stack, depth, b->buf + strlen(b->buf),
static_cast<int>(sizeof(b->buf) - strlen(b->buf)),
symbolize);
ABSL_RAW_LOG(ERROR, "%s", b->buf);
}
if (path_len > static_cast<int>(ABSL_ARRAYSIZE(b->path))) {
ABSL_RAW_LOG(ERROR, "(long cycle; list truncated)");
}
if (synch_deadlock_detection.load(std::memory_order_acquire) ==
OnDeadlockCycle::kAbort) {
deadlock_graph_mu.Unlock(); // avoid deadlock in fatal sighandler
ABSL_RAW_LOG(FATAL, "dying due to potential deadlock");
return mu_id;
}
break; // report at most one potential deadlock per acquisition
}
}
return mu_id;
}
// Invoke DeadlockCheck() iff we're in debug mode and
// deadlock checking has been enabled.
static inline GraphId DebugOnlyDeadlockCheck(Mutex* mu) {
if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
return DeadlockCheck(mu);
} else {
return InvalidGraphId();
}
}
void Mutex::ForgetDeadlockInfo() {
if (kDebugMode && synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
deadlock_graph_mu.Lock();
if (deadlock_graph != nullptr) {
deadlock_graph->RemoveNode(this);
}
deadlock_graph_mu.Unlock();
}
}
void Mutex::AssertNotHeld() const {
// We have the data to allow this check only if in debug mode and deadlock
// detection is enabled.
if (kDebugMode &&
(mu_.load(std::memory_order_relaxed) & (kMuWriter | kMuReader)) != 0 &&
synch_deadlock_detection.load(std::memory_order_acquire) !=
OnDeadlockCycle::kIgnore) {
GraphId id = GetGraphId(const_cast<Mutex*>(this));
SynchLocksHeld* locks = Synch_GetAllLocks();
for (int i = 0; i != locks->n; i++) {
if (locks->locks[i].id == id) {
SynchEvent* mu_events = GetSynchEvent(this);
ABSL_RAW_LOG(FATAL, "thread should not hold mutex %p %s",
static_cast<const void*>(this),
(mu_events == nullptr ? "" : mu_events->name));
}
}
}
}
// Attempt to acquire *mu, and return whether successful. The implementation
// may spin for a short while if the lock cannot be acquired immediately.
static bool TryAcquireWithSpinning(std::atomic<intptr_t>* mu) {
int c = globals.spinloop_iterations.load(std::memory_order_relaxed);
do { // do/while somewhat faster on AMD
intptr_t v = mu->load(std::memory_order_relaxed);
if ((v & (kMuReader | kMuEvent)) != 0) {
return false; // a reader or tracing -> give up
} else if (((v & kMuWriter) == 0) && // no holder -> try to acquire
mu->compare_exchange_strong(v, kMuWriter | v,
std::memory_order_acquire,
std::memory_order_relaxed)) {
return true;
}
} while (--c > 0);
return false;
}
void Mutex::Lock() {
ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
GraphId id = DebugOnlyDeadlockCheck(this);
intptr_t v = mu_.load(std::memory_order_relaxed);
// try fast acquire, then spin loop
if (ABSL_PREDICT_FALSE((v & (kMuWriter | kMuReader | kMuEvent)) != 0) ||
ABSL_PREDICT_FALSE(!mu_.compare_exchange_strong(
v, kMuWriter | v, std::memory_order_acquire,
std::memory_order_relaxed))) {
// try spin acquire, then slow loop
if (ABSL_PREDICT_FALSE(!TryAcquireWithSpinning(&this->mu_))) {
this->LockSlow(kExclusive, nullptr, 0);
}
}
DebugOnlyLockEnter(this, id);
ABSL_TSAN_MUTEX_POST_LOCK(this, 0, 0);
}
void Mutex::ReaderLock() {
ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_read_lock);
GraphId id = DebugOnlyDeadlockCheck(this);
intptr_t v = mu_.load(std::memory_order_relaxed);
for (;;) {
// If there are non-readers holding the lock, use the slow loop.
if (ABSL_PREDICT_FALSE(v & (kMuWriter | kMuWait | kMuEvent)) != 0) {
this->LockSlow(kShared, nullptr, 0);
break;
}
// We can avoid the loop and only use the CAS when the lock is free or
// only held by readers.
if (ABSL_PREDICT_TRUE(mu_.compare_exchange_weak(
v, (kMuReader | v) + kMuOne, std::memory_order_acquire,
std::memory_order_relaxed))) {
break;
}
}
DebugOnlyLockEnter(this, id);
ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_read_lock, 0);
}
bool Mutex::LockWhenCommon(const Condition& cond,
synchronization_internal::KernelTimeout t,
bool write) {
MuHow how = write ? kExclusive : kShared;
ABSL_TSAN_MUTEX_PRE_LOCK(this, TsanFlags(how));
GraphId id = DebugOnlyDeadlockCheck(this);
bool res = LockSlowWithDeadline(how, &cond, t, 0);
DebugOnlyLockEnter(this, id);
ABSL_TSAN_MUTEX_POST_LOCK(this, TsanFlags(how), 0);
return res;
}
bool Mutex::AwaitCommon(const Condition& cond, KernelTimeout t) {
if (kDebugMode) {
this->AssertReaderHeld();
}
if (cond.Eval()) { // condition already true; nothing to do
return true;
}
MuHow how =
(mu_.load(std::memory_order_relaxed) & kMuWriter) ? kExclusive : kShared;
ABSL_TSAN_MUTEX_PRE_UNLOCK(this, TsanFlags(how));
SynchWaitParams waitp(how, &cond, t, nullptr /*no cvmu*/,
Synch_GetPerThreadAnnotated(this),
nullptr /*no cv_word*/);
this->UnlockSlow(&waitp);
this->Block(waitp.thread);
ABSL_TSAN_MUTEX_POST_UNLOCK(this, TsanFlags(how));
ABSL_TSAN_MUTEX_PRE_LOCK(this, TsanFlags(how));
this->LockSlowLoop(&waitp, kMuHasBlocked | kMuIsCond);
bool res = waitp.cond != nullptr || // => cond known true from LockSlowLoop
EvalConditionAnnotated(&cond, this, true, false, how == kShared);
ABSL_TSAN_MUTEX_POST_LOCK(this, TsanFlags(how), 0);
ABSL_RAW_CHECK(res || t.has_timeout(),
"condition untrue on return from Await");
return res;
}
bool Mutex::TryLock() {
ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
intptr_t v = mu_.load(std::memory_order_relaxed);
// Try fast acquire.
if (ABSL_PREDICT_TRUE((v & (kMuWriter | kMuReader | kMuEvent)) == 0)) {
if (ABSL_PREDICT_TRUE(mu_.compare_exchange_strong(
v, kMuWriter | v, std::memory_order_acquire,
std::memory_order_relaxed))) {
DebugOnlyLockEnter(this);
ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
return true;
}
} else if (ABSL_PREDICT_FALSE((v & kMuEvent) != 0)) {
// We're recording events.
return TryLockSlow();
}
ABSL_TSAN_MUTEX_POST_LOCK(
this, __tsan_mutex_try_lock | __tsan_mutex_try_lock_failed, 0);
return false;
}
ABSL_ATTRIBUTE_NOINLINE bool Mutex::TryLockSlow() {
intptr_t v = mu_.load(std::memory_order_relaxed);
if ((v & kExclusive->slow_need_zero) == 0 && // try fast acquire
mu_.compare_exchange_strong(
v, (kExclusive->fast_or | v) + kExclusive->fast_add,
std::memory_order_acquire, std::memory_order_relaxed)) {
DebugOnlyLockEnter(this);
PostSynchEvent(this, SYNCH_EV_TRYLOCK_SUCCESS);
ABSL_TSAN_MUTEX_POST_LOCK(this, __tsan_mutex_try_lock, 0);
return true;
}
PostSynchEvent(this, SYNCH_EV_TRYLOCK_FAILED);
ABSL_TSAN_MUTEX_POST_LOCK(
this, __tsan_mutex_try_lock | __tsan_mutex_try_lock_failed, 0);
return false;
}
bool Mutex::ReaderTryLock() {
ABSL_TSAN_MUTEX_PRE_LOCK(this,
__tsan_mutex_read_lock | __tsan_mutex_try_lock);
intptr_t v = mu_.load(std::memory_order_relaxed);
// Clang tends to unroll the loop when compiling with optimization.
// But in this case it just unnecessary increases code size.
// If CAS is failing due to contention, the jump cost is negligible.
#if defined(__clang__)
#pragma nounroll
#endif
// The while-loops (here and below) iterate only if the mutex word keeps
// changing (typically because the reader count changes) under the CAS.
// We limit the number of attempts to avoid having to think about livelock.
for (int loop_limit = 5; loop_limit != 0; loop_limit--) {
if (ABSL_PREDICT_FALSE((v & (kMuWriter | kMuWait | kMuEvent)) != 0)) {
break;
}
if (ABSL_PREDICT_TRUE(mu_.compare_exchange_strong(
v, (kMuReader | v) + kMuOne, std::memory_order_acquire,
std::memory_order_relaxed))) {
DebugOnlyLockEnter(this);
ABSL_TSAN_MUTEX_POST_LOCK(
this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
return true;
}
}
if (ABSL_PREDICT_TRUE((v & kMuEvent) == 0)) {
ABSL_TSAN_MUTEX_POST_LOCK(this,
__tsan_mutex_read_lock | __tsan_mutex_try_lock |
__tsan_mutex_try_lock_failed,
0);
return false;
}
// we're recording events
return ReaderTryLockSlow();
}
ABSL_ATTRIBUTE_NOINLINE bool Mutex::ReaderTryLockSlow() {
intptr_t v = mu_.load(std::memory_order_relaxed);
#if defined(__clang__)
#pragma nounroll
#endif
for (int loop_limit = 5; loop_limit != 0; loop_limit--) {
if ((v & kShared->slow_need_zero) == 0 &&
mu_.compare_exchange_strong(v, (kMuReader | v) + kMuOne,
std::memory_order_acquire,
std::memory_order_relaxed)) {
DebugOnlyLockEnter(this);
PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_SUCCESS);
ABSL_TSAN_MUTEX_POST_LOCK(
this, __tsan_mutex_read_lock | __tsan_mutex_try_lock, 0);
return true;
}
}
PostSynchEvent(this, SYNCH_EV_READERTRYLOCK_FAILED);
ABSL_TSAN_MUTEX_POST_LOCK(this,
__tsan_mutex_read_lock | __tsan_mutex_try_lock |
__tsan_mutex_try_lock_failed,
0);
return false;
}
void Mutex::Unlock() {
ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
DebugOnlyLockLeave(this);
intptr_t v = mu_.load(std::memory_order_relaxed);
if (kDebugMode && ((v & (kMuWriter | kMuReader)) != kMuWriter)) {
ABSL_RAW_LOG(FATAL, "Mutex unlocked when destroyed or not locked: v=0x%x",
static_cast<unsigned>(v));
}
// should_try_cas is whether we'll try a compare-and-swap immediately.
// NOTE: optimized out when kDebugMode is false.
bool should_try_cas = ((v & (kMuEvent | kMuWriter)) == kMuWriter &&
(v & (kMuWait | kMuDesig)) != kMuWait);
// But, we can use an alternate computation of it, that compilers
// currently don't find on their own. When that changes, this function
// can be simplified.
intptr_t x = (v ^ (kMuWriter | kMuWait)) & (kMuWriter | kMuEvent);
intptr_t y = (v ^ (kMuWriter | kMuWait)) & (kMuWait | kMuDesig);
// Claim: "x == 0 && y > 0" is equal to should_try_cas.
// Also, because kMuWriter and kMuEvent exceed kMuDesig and kMuWait,
// all possible non-zero values for x exceed all possible values for y.
// Therefore, (x == 0 && y > 0) == (x < y).
if (kDebugMode && should_try_cas != (x < y)) {
// We would usually use PRIdPTR here, but is not correctly implemented
// within the android toolchain.
ABSL_RAW_LOG(FATAL, "internal logic error %llx %llx %llx\n",
static_cast<long long>(v), static_cast<long long>(x),
static_cast<long long>(y));
}
if (x < y && mu_.compare_exchange_strong(v, v & ~(kMuWrWait | kMuWriter),
std::memory_order_release,
std::memory_order_relaxed)) {
// fast writer release (writer with no waiters or with designated waker)
} else {
this->UnlockSlow(nullptr /*no waitp*/); // take slow path
}
ABSL_TSAN_MUTEX_POST_UNLOCK(this, 0);
}
// Requires v to represent a reader-locked state.
static bool ExactlyOneReader(intptr_t v) {
assert((v & (kMuWriter | kMuReader)) == kMuReader);
assert((v & kMuHigh) != 0);
// The more straightforward "(v & kMuHigh) == kMuOne" also works, but
// on some architectures the following generates slightly smaller code.
// It may be faster too.
constexpr intptr_t kMuMultipleWaitersMask = kMuHigh ^ kMuOne;
return (v & kMuMultipleWaitersMask) == 0;
}
void Mutex::ReaderUnlock() {
ABSL_TSAN_MUTEX_PRE_UNLOCK(this, __tsan_mutex_read_lock);
DebugOnlyLockLeave(this);
intptr_t v = mu_.load(std::memory_order_relaxed);
assert((v & (kMuWriter | kMuReader)) == kMuReader);
for (;;) {
if (ABSL_PREDICT_FALSE((v & (kMuReader | kMuWait | kMuEvent)) !=
kMuReader)) {
this->UnlockSlow(nullptr /*no waitp*/); // take slow path
break;
}
// fast reader release (reader with no waiters)
intptr_t clear = ExactlyOneReader(v) ? kMuReader | kMuOne : kMuOne;
if (ABSL_PREDICT_TRUE(
mu_.compare_exchange_strong(v, v - clear, std::memory_order_release,
std::memory_order_relaxed))) {
break;
}
}
ABSL_TSAN_MUTEX_POST_UNLOCK(this, __tsan_mutex_read_lock);
}
// Clears the designated waker flag in the mutex if this thread has blocked, and
// therefore may be the designated waker.
static intptr_t ClearDesignatedWakerMask(int flag) {
assert(flag >= 0);
assert(flag <= 1);
switch (flag) {
case 0: // not blocked
return ~static_cast<intptr_t>(0);
case 1: // blocked; turn off the designated waker bit
return ~static_cast<intptr_t>(kMuDesig);
}
ABSL_UNREACHABLE();
}
// Conditionally ignores the existence of waiting writers if a reader that has
// already blocked once wakes up.
static intptr_t IgnoreWaitingWritersMask(int flag) {
assert(flag >= 0);
assert(flag <= 1);
switch (flag) {
case 0: // not blocked
return ~static_cast<intptr_t>(0);
case 1: // blocked; pretend there are no waiting writers
return ~static_cast<intptr_t>(kMuWrWait);
}
ABSL_UNREACHABLE();
}
// Internal version of LockWhen(). See LockSlowWithDeadline()
ABSL_ATTRIBUTE_NOINLINE void Mutex::LockSlow(MuHow how, const Condition* cond,
int flags) {
// Note: we specifically initialize spinloop_iterations after the first use
// in TryAcquireWithSpinning so that Lock function does not have any non-tail
// calls and consequently a stack frame. It's fine to have spinloop_iterations
// uninitialized (meaning no spinning) in all initial uncontended Lock calls
// and in the first contended call. After that we will have
// spinloop_iterations properly initialized.
if (ABSL_PREDICT_FALSE(
globals.spinloop_iterations.load(std::memory_order_relaxed) == 0)) {
if (absl::base_internal::NumCPUs() > 1) {
// If this is multiprocessor, allow spinning.
globals.spinloop_iterations.store(1500, std::memory_order_relaxed);
} else {
// If this a uniprocessor, only yield/sleep.
globals.spinloop_iterations.store(-1, std::memory_order_relaxed);
}
}
ABSL_RAW_CHECK(
this->LockSlowWithDeadline(how, cond, KernelTimeout::Never(), flags),
"condition untrue on return from LockSlow");
}
// Compute cond->Eval() and tell race detectors that we do it under mutex mu.
static inline bool EvalConditionAnnotated(const Condition* cond, Mutex* mu,
bool locking, bool trylock,
bool read_lock) {
// Delicate annotation dance.
// We are currently inside of read/write lock/unlock operation.
// All memory accesses are ignored inside of mutex operations + for unlock
// operation tsan considers that we've already released the mutex.
bool res = false;
#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
const uint32_t flags = read_lock ? __tsan_mutex_read_lock : 0;
const uint32_t tryflags = flags | (trylock ? __tsan_mutex_try_lock : 0);
#endif
if (locking) {
// For lock we pretend that we have finished the operation,
// evaluate the predicate, then unlock the mutex and start locking it again
// to match the annotation at the end of outer lock operation.
// Note: we can't simply do POST_LOCK, Eval, PRE_LOCK, because then tsan
// will think the lock acquisition is recursive which will trigger
// deadlock detector.
ABSL_TSAN_MUTEX_POST_LOCK(mu, tryflags, 0);
res = cond->Eval();
// There is no "try" version of Unlock, so use flags instead of tryflags.
ABSL_TSAN_MUTEX_PRE_UNLOCK(mu, flags);
ABSL_TSAN_MUTEX_POST_UNLOCK(mu, flags);
ABSL_TSAN_MUTEX_PRE_LOCK(mu, tryflags);
} else {
// Similarly, for unlock we pretend that we have unlocked the mutex,
// lock the mutex, evaluate the predicate, and start unlocking it again
// to match the annotation at the end of outer unlock operation.
ABSL_TSAN_MUTEX_POST_UNLOCK(mu, flags);
ABSL_TSAN_MUTEX_PRE_LOCK(mu, flags);
ABSL_TSAN_MUTEX_POST_LOCK(mu, flags, 0);
res = cond->Eval();
ABSL_TSAN_MUTEX_PRE_UNLOCK(mu, flags);
}
// Prevent unused param warnings in non-TSAN builds.
static_cast<void>(mu);
static_cast<void>(trylock);
static_cast<void>(read_lock);
return res;
}
// Compute cond->Eval() hiding it from race detectors.
// We are hiding it because inside of UnlockSlow we can evaluate a predicate
// that was just added by a concurrent Lock operation; Lock adds the predicate
// to the internal Mutex list without actually acquiring the Mutex
// (it only acquires the internal spinlock, which is rightfully invisible for
// tsan). As the result there is no tsan-visible synchronization between the
// addition and this thread. So if we would enable race detection here,
// it would race with the predicate initialization.
static inline bool EvalConditionIgnored(Mutex* mu, const Condition* cond) {
// Memory accesses are already ignored inside of lock/unlock operations,
// but synchronization operations are also ignored. When we evaluate the
// predicate we must ignore only memory accesses but not synchronization,
// because missed synchronization can lead to false reports later.
// So we "divert" (which un-ignores both memory accesses and synchronization)
// and then separately turn on ignores of memory accesses.
ABSL_TSAN_MUTEX_PRE_DIVERT(mu, 0);
ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
bool res = cond->Eval();
ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
ABSL_TSAN_MUTEX_POST_DIVERT(mu, 0);
static_cast<void>(mu); // Prevent unused param warning in non-TSAN builds.
return res;
}
// Internal equivalent of *LockWhenWithDeadline(), where
// "t" represents the absolute timeout; !t.has_timeout() means "forever".
// "how" is "kShared" (for ReaderLockWhen) or "kExclusive" (for LockWhen)
// In flags, bits are ored together:
// - kMuHasBlocked indicates that the client has already blocked on the call so
// the designated waker bit must be cleared and waiting writers should not
// obstruct this call
// - kMuIsCond indicates that this is a conditional acquire (condition variable,
// Await, LockWhen) so contention profiling should be suppressed.
bool Mutex::LockSlowWithDeadline(MuHow how, const Condition* cond,
KernelTimeout t, int flags) {
intptr_t v = mu_.load(std::memory_order_relaxed);
bool unlock = false;
if ((v & how->fast_need_zero) == 0 && // try fast acquire
mu_.compare_exchange_strong(
v,
(how->fast_or |
(v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
how->fast_add,
std::memory_order_acquire, std::memory_order_relaxed)) {
if (cond == nullptr ||
EvalConditionAnnotated(cond, this, true, false, how == kShared)) {
return true;
}
unlock = true;
}
SynchWaitParams waitp(how, cond, t, nullptr /*no cvmu*/,
Synch_GetPerThreadAnnotated(this),
nullptr /*no cv_word*/);
if (cond != nullptr) {
flags |= kMuIsCond;
}
if (unlock) {
this->UnlockSlow(&waitp);
this->Block(waitp.thread);
flags |= kMuHasBlocked;
}
this->LockSlowLoop(&waitp, flags);
return waitp.cond != nullptr || // => cond known true from LockSlowLoop
cond == nullptr ||
EvalConditionAnnotated(cond, this, true, false, how == kShared);
}
// RAW_CHECK_FMT() takes a condition, a printf-style format string, and
// the printf-style argument list. The format string must be a literal.
// Arguments after the first are not evaluated unless the condition is true.
#define RAW_CHECK_FMT(cond, ...) \
do { \
if (ABSL_PREDICT_FALSE(!(cond))) { \
ABSL_RAW_LOG(FATAL, "Check " #cond " failed: " __VA_ARGS__); \
} \
} while (0)
static void CheckForMutexCorruption(intptr_t v, const char* label) {
// Test for either of two situations that should not occur in v:
// kMuWriter and kMuReader
// kMuWrWait and !kMuWait
const uintptr_t w = static_cast<uintptr_t>(v ^ kMuWait);
// By flipping that bit, we can now test for:
// kMuWriter and kMuReader in w
// kMuWrWait and kMuWait in w
// We've chosen these two pairs of values to be so that they will overlap,
// respectively, when the word is left shifted by three. This allows us to
// save a branch in the common (correct) case of them not being coincident.
static_assert(kMuReader << 3 == kMuWriter, "must match");
static_assert(kMuWait << 3 == kMuWrWait, "must match");
if (ABSL_PREDICT_TRUE((w & (w << 3) & (kMuWriter | kMuWrWait)) == 0)) return;
RAW_CHECK_FMT((v & (kMuWriter | kMuReader)) != (kMuWriter | kMuReader),
"%s: Mutex corrupt: both reader and writer lock held: %p",
label, reinterpret_cast<void*>(v));
RAW_CHECK_FMT((v & (kMuWait | kMuWrWait)) != kMuWrWait,
"%s: Mutex corrupt: waiting writer with no waiters: %p", label,
reinterpret_cast<void*>(v));
assert(false);
}
void Mutex::LockSlowLoop(SynchWaitParams* waitp, int flags) {
SchedulingGuard::ScopedDisable disable_rescheduling;
int c = 0;
intptr_t v = mu_.load(std::memory_order_relaxed);
if ((v & kMuEvent) != 0) {
PostSynchEvent(
this, waitp->how == kExclusive ? SYNCH_EV_LOCK : SYNCH_EV_READERLOCK);
}
ABSL_RAW_CHECK(
waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
"detected illegal recursion into Mutex code");
for (;;) {
v = mu_.load(std::memory_order_relaxed);
CheckForMutexCorruption(v, "Lock");
if ((v & waitp->how->slow_need_zero) == 0) {
if (mu_.compare_exchange_strong(
v,
(waitp->how->fast_or |
(v & ClearDesignatedWakerMask(flags & kMuHasBlocked))) +
waitp->how->fast_add,
std::memory_order_acquire, std::memory_order_relaxed)) {
if (waitp->cond == nullptr ||
EvalConditionAnnotated(waitp->cond, this, true, false,
waitp->how == kShared)) {
break; // we timed out, or condition true, so return
}
this->UnlockSlow(waitp); // got lock but condition false
this->Block(waitp->thread);
flags |= kMuHasBlocked;
c = 0;
}
} else { // need to access waiter list
bool dowait = false;
if ((v & (kMuSpin | kMuWait)) == 0) { // no waiters
// This thread tries to become the one and only waiter.
PerThreadSynch* new_h = Enqueue(nullptr, waitp, v, flags);
intptr_t nv =
(v & ClearDesignatedWakerMask(flags & kMuHasBlocked) & kMuLow) |
kMuWait;
ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to empty list failed");
if (waitp->how == kExclusive && (v & kMuReader) != 0) {
nv |= kMuWrWait;
}
if (mu_.compare_exchange_strong(
v, reinterpret_cast<intptr_t>(new_h) | nv,
std::memory_order_release, std::memory_order_relaxed)) {
dowait = true;
} else { // attempted Enqueue() failed
// zero out the waitp field set by Enqueue()
waitp->thread->waitp = nullptr;
}
} else if ((v & waitp->how->slow_inc_need_zero &
IgnoreWaitingWritersMask(flags & kMuHasBlocked)) == 0) {
// This is a reader that needs to increment the reader count,
// but the count is currently held in the last waiter.
if (mu_.compare_exchange_strong(
v,
(v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
kMuSpin | kMuReader,
std::memory_order_acquire, std::memory_order_relaxed)) {
PerThreadSynch* h = GetPerThreadSynch(v);
h->readers += kMuOne; // inc reader count in waiter
do { // release spinlock
v = mu_.load(std::memory_order_relaxed);
} while (!mu_.compare_exchange_weak(v, (v & ~kMuSpin) | kMuReader,
std::memory_order_release,
std::memory_order_relaxed));
if (waitp->cond == nullptr ||
EvalConditionAnnotated(waitp->cond, this, true, false,
waitp->how == kShared)) {
break; // we timed out, or condition true, so return
}
this->UnlockSlow(waitp); // got lock but condition false
this->Block(waitp->thread);
flags |= kMuHasBlocked;
c = 0;
}
} else if ((v & kMuSpin) == 0 && // attempt to queue ourselves
mu_.compare_exchange_strong(
v,
(v & ClearDesignatedWakerMask(flags & kMuHasBlocked)) |
kMuSpin | kMuWait,
std::memory_order_acquire, std::memory_order_relaxed)) {
PerThreadSynch* h = GetPerThreadSynch(v);
PerThreadSynch* new_h = Enqueue(h, waitp, v, flags);
intptr_t wr_wait = 0;
ABSL_RAW_CHECK(new_h != nullptr, "Enqueue to list failed");
if (waitp->how == kExclusive && (v & kMuReader) != 0) {
wr_wait = kMuWrWait; // give priority to a waiting writer
}
do { // release spinlock
v = mu_.load(std::memory_order_relaxed);
} while (!mu_.compare_exchange_weak(
v,
(v & (kMuLow & ~kMuSpin)) | kMuWait | wr_wait |
reinterpret_cast<intptr_t>(new_h),
std::memory_order_release, std::memory_order_relaxed));
dowait = true;
}
if (dowait) {
this->Block(waitp->thread); // wait until removed from list or timeout
flags |= kMuHasBlocked;
c = 0;
}
}
ABSL_RAW_CHECK(
waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
"detected illegal recursion into Mutex code");
// delay, then try again
c = synchronization_internal::MutexDelay(c, GENTLE);
}
ABSL_RAW_CHECK(
waitp->thread->waitp == nullptr || waitp->thread->suppress_fatal_errors,
"detected illegal recursion into Mutex code");
if ((v & kMuEvent) != 0) {
PostSynchEvent(this, waitp->how == kExclusive
? SYNCH_EV_LOCK_RETURNING
: SYNCH_EV_READERLOCK_RETURNING);
}
}
// Unlock this mutex, which is held by the current thread.
// If waitp is non-zero, it must be the wait parameters for the current thread
// which holds the lock but is not runnable because its condition is false
// or it is in the process of blocking on a condition variable; it must requeue
// itself on the mutex/condvar to wait for its condition to become true.
ABSL_ATTRIBUTE_NOINLINE void Mutex::UnlockSlow(SynchWaitParams* waitp) {
SchedulingGuard::ScopedDisable disable_rescheduling;
intptr_t v = mu_.load(std::memory_order_relaxed);
this->AssertReaderHeld();
CheckForMutexCorruption(v, "Unlock");
if ((v & kMuEvent) != 0) {
PostSynchEvent(
this, (v & kMuWriter) != 0 ? SYNCH_EV_UNLOCK : SYNCH_EV_READERUNLOCK);
}
int c = 0;
// the waiter under consideration to wake, or zero
PerThreadSynch* w = nullptr;
// the predecessor to w or zero
PerThreadSynch* pw = nullptr;
// head of the list searched previously, or zero
PerThreadSynch* old_h = nullptr;
// a condition that's known to be false.
PerThreadSynch* wake_list = kPerThreadSynchNull; // list of threads to wake
intptr_t wr_wait = 0; // set to kMuWrWait if we wake a reader and a
// later writer could have acquired the lock
// (starvation avoidance)
ABSL_RAW_CHECK(waitp == nullptr || waitp->thread->waitp == nullptr ||
waitp->thread->suppress_fatal_errors,
"detected illegal recursion into Mutex code");
// This loop finds threads wake_list to wakeup if any, and removes them from
// the list of waiters. In addition, it places waitp.thread on the queue of
// waiters if waitp is non-zero.
for (;;) {
v = mu_.load(std::memory_order_relaxed);
if ((v & kMuWriter) != 0 && (v & (kMuWait | kMuDesig)) != kMuWait &&
waitp == nullptr) {
// fast writer release (writer with no waiters or with designated waker)
if (mu_.compare_exchange_strong(v, v & ~(kMuWrWait | kMuWriter),
std::memory_order_release,
std::memory_order_relaxed)) {
return;
}
} else if ((v & (kMuReader | kMuWait)) == kMuReader && waitp == nullptr) {
// fast reader release (reader with no waiters)
intptr_t clear = ExactlyOneReader(v) ? kMuReader | kMuOne : kMuOne;
if (mu_.compare_exchange_strong(v, v - clear, std::memory_order_release,
std::memory_order_relaxed)) {
return;
}
} else if ((v & kMuSpin) == 0 && // attempt to get spinlock
mu_.compare_exchange_strong(v, v | kMuSpin,
std::memory_order_acquire,
std::memory_order_relaxed)) {
if ((v & kMuWait) == 0) { // no one to wake
intptr_t nv;
bool do_enqueue = true; // always Enqueue() the first time
ABSL_RAW_CHECK(waitp != nullptr,
"UnlockSlow is confused"); // about to sleep
do { // must loop to release spinlock as reader count may change
v = mu_.load(std::memory_order_relaxed);
// decrement reader count if there are readers
intptr_t new_readers = (v >= kMuOne) ? v - kMuOne : v;
PerThreadSynch* new_h = nullptr;
if (do_enqueue) {
// If we are enqueuing on a CondVar (waitp->cv_word != nullptr) then
// we must not retry here. The initial attempt will always have
// succeeded, further attempts would enqueue us against *this due to
// Fer() handling.
do_enqueue = (waitp->cv_word == nullptr);
new_h = Enqueue(nullptr, waitp, new_readers, kMuIsCond);
}
intptr_t clear = kMuWrWait | kMuWriter; // by default clear write bit
if ((v & kMuWriter) == 0 && ExactlyOneReader(v)) { // last reader
clear = kMuWrWait | kMuReader; // clear read bit
}
nv = (v & kMuLow & ~clear & ~kMuSpin);
if (new_h != nullptr) {
nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
} else { // new_h could be nullptr if we queued ourselves on a
// CondVar
// In that case, we must place the reader count back in the mutex
// word, as Enqueue() did not store it in the new waiter.
nv |= new_readers & kMuHigh;
}
// release spinlock & our lock; retry if reader-count changed
// (writer count cannot change since we hold lock)
} while (!mu_.compare_exchange_weak(v, nv, std::memory_order_release,
std::memory_order_relaxed));
break;
}
// There are waiters.
// Set h to the head of the circular waiter list.
PerThreadSynch* h = GetPerThreadSynch(v);
if ((v & kMuReader) != 0 && (h->readers & kMuHigh) > kMuOne) {
// a reader but not the last
h->readers -= kMuOne; // release our lock
intptr_t nv = v; // normally just release spinlock
if (waitp != nullptr) { // but waitp!=nullptr => must queue ourselves
PerThreadSynch* new_h = Enqueue(h, waitp, v, kMuIsCond);
ABSL_RAW_CHECK(new_h != nullptr,
"waiters disappeared during Enqueue()!");
nv &= kMuLow;
nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
}
mu_.store(nv, std::memory_order_release); // release spinlock
// can release with a store because there were waiters
break;
}
// Either we didn't search before, or we marked the queue
// as "maybe_unlocking" and no one else should have changed it.
ABSL_RAW_CHECK(old_h == nullptr || h->maybe_unlocking,
"Mutex queue changed beneath us");
// The lock is becoming free, and there's a waiter
if (old_h != nullptr &&
!old_h->may_skip) { // we used old_h as a terminator
old_h->may_skip = true; // allow old_h to skip once more
ABSL_RAW_CHECK(old_h->skip == nullptr, "illegal skip from head");
if (h != old_h && MuEquivalentWaiter(old_h, old_h->next)) {
old_h->skip = old_h->next; // old_h not head & can skip to successor
}
}
if (h->next->waitp->how == kExclusive &&
h->next->waitp->cond == nullptr) {
// easy case: writer with no condition; no need to search
pw = h; // wake w, the successor of h (=pw)
w = h->next;
w->wake = true;
// We are waking up a writer. This writer may be racing against
// an already awake reader for the lock. We want the
// writer to usually win this race,
// because if it doesn't, we can potentially keep taking a reader
// perpetually and writers will starve. Worse than
// that, this can also starve other readers if kMuWrWait gets set
// later.
wr_wait = kMuWrWait;
} else if (w != nullptr && (w->waitp->how == kExclusive || h == old_h)) {
// we found a waiter w to wake on a previous iteration and either it's
// a writer, or we've searched the entire list so we have all the
// readers.
if (pw == nullptr) { // if w's predecessor is unknown, it must be h
pw = h;
}
} else {
// At this point we don't know all the waiters to wake, and the first
// waiter has a condition or is a reader. We avoid searching over
// waiters we've searched on previous iterations by starting at
// old_h if it's set. If old_h==h, there's no one to wakeup at all.
if (old_h == h) { // we've searched before, and nothing's new
// so there's no one to wake.
intptr_t nv = (v & ~(kMuReader | kMuWriter | kMuWrWait));
h->readers = 0;
h->maybe_unlocking = false; // finished unlocking
if (waitp != nullptr) { // we must queue ourselves and sleep
PerThreadSynch* new_h = Enqueue(h, waitp, v, kMuIsCond);
nv &= kMuLow;
if (new_h != nullptr) {
nv |= kMuWait | reinterpret_cast<intptr_t>(new_h);
} // else new_h could be nullptr if we queued ourselves on a
// CondVar
}
// release spinlock & lock
// can release with a store because there were waiters
mu_.store(nv, std::memory_order_release);
break;
}
// set up to walk the list
PerThreadSynch* w_walk; // current waiter during list walk
PerThreadSynch* pw_walk; // previous waiter during list walk
if (old_h != nullptr) { // we've searched up to old_h before
pw_walk = old_h;
w_walk = old_h->next;
} else { // no prior search, start at beginning
pw_walk =
nullptr; // h->next's predecessor may change; don't record it
w_walk = h->next;
}
h->may_skip = false; // ensure we never skip past h in future searches
// even if other waiters are queued after it.
ABSL_RAW_CHECK(h->skip == nullptr, "illegal skip from head");
h->maybe_unlocking = true; // we're about to scan the waiter list
// without the spinlock held.
// Enqueue must be conservative about
// priority queuing.
// We must release the spinlock to evaluate the conditions.
mu_.store(v, std::memory_order_release); // release just spinlock
// can release with a store because there were waiters
// h is the last waiter queued, and w_walk the first unsearched waiter.
// Without the spinlock, the locations mu_ and h->next may now change
// underneath us, but since we hold the lock itself, the only legal
// change is to add waiters between h and w_walk. Therefore, it's safe
// to walk the path from w_walk to h inclusive. (TryRemove() can remove
// a waiter anywhere, but it acquires both the spinlock and the Mutex)
old_h = h; // remember we searched to here
// Walk the path upto and including h looking for waiters we can wake.
while (pw_walk != h) {
w_walk->wake = false;
if (w_walk->waitp->cond ==
nullptr || // no condition => vacuously true OR
// this thread's condition is true
EvalConditionIgnored(this, w_walk->waitp->cond)) {
if (w == nullptr) {
w_walk->wake = true; // can wake this waiter
w = w_walk;
pw = pw_walk;
if (w_walk->waitp->how == kExclusive) {
wr_wait = kMuWrWait;
break; // bail if waking this writer
}
} else if (w_walk->waitp->how == kShared) { // wake if a reader
w_walk->wake = true;
} else { // writer with true condition
wr_wait = kMuWrWait;
}
}
if (w_walk->wake) { // we're waking reader w_walk
pw_walk = w_walk; // don't skip similar waiters
} else { // not waking; skip as much as possible
pw_walk = Skip(w_walk);
}
// If pw_walk == h, then load of pw_walk->next can race with
// concurrent write in Enqueue(). However, at the same time
// we do not need to do the load, because we will bail out
// from the loop anyway.
if (pw_walk != h) {
w_walk = pw_walk->next;
}
}
continue; // restart for(;;)-loop to wakeup w or to find more waiters
}
ABSL_RAW_CHECK(pw->next == w, "pw not w's predecessor");
// The first (and perhaps only) waiter we've chosen to wake is w, whose
// predecessor is pw. If w is a reader, we must wake all the other
// waiters with wake==true as well. We may also need to queue
// ourselves if waitp != null. The spinlock and the lock are still
// held.
// This traverses the list in [ pw->next, h ], where h is the head,
// removing all elements with wake==true and placing them in the
// singly-linked list wake_list. Returns the new head.
h = DequeueAllWakeable(h, pw, &wake_list);
intptr_t nv = (v & kMuEvent) | kMuDesig;
// assume no waiters left,
// set kMuDesig for INV1a
if (waitp != nullptr) { // we must queue ourselves and sleep
h = Enqueue(h, waitp, v, kMuIsCond);
// h is new last waiter; could be null if we queued ourselves on a
// CondVar
}
ABSL_RAW_CHECK(wake_list != kPerThreadSynchNull,
"unexpected empty wake list");
if (h != nullptr) { // there are waiters left
h->readers = 0;
h->maybe_unlocking = false; // finished unlocking
nv |= wr_wait | kMuWait | reinterpret_cast<intptr_t>(h);
}
// release both spinlock & lock
// can release with a store because there were waiters
mu_.store(nv, std::memory_order_release);
break; // out of for(;;)-loop
}
// aggressive here; no one can proceed till we do
c = synchronization_internal::MutexDelay(c, AGGRESSIVE);
} // end of for(;;)-loop
if (wake_list != kPerThreadSynchNull) {
int64_t total_wait_cycles = 0;
int64_t max_wait_cycles = 0;
int64_t now = CycleClock::Now();
do {
// Profile lock contention events only if the waiter was trying to acquire
// the lock, not waiting on a condition variable or Condition.
if (!wake_list->cond_waiter) {
int64_t cycles_waited =
(now - wake_list->waitp->contention_start_cycles);
total_wait_cycles += cycles_waited;
if (max_wait_cycles == 0) max_wait_cycles = cycles_waited;
wake_list->waitp->contention_start_cycles = now;
wake_list->waitp->should_submit_contention_data = true;
}
wake_list = Wakeup(wake_list); // wake waiters
} while (wake_list != kPerThreadSynchNull);
if (total_wait_cycles > 0) {
mutex_tracer("slow release", this, total_wait_cycles);
ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0);
submit_profile_data(total_wait_cycles);
ABSL_TSAN_MUTEX_POST_DIVERT(this, 0);
}
}
}
// Used by CondVar implementation to reacquire mutex after waking from
// condition variable. This routine is used instead of Lock() because the
// waiting thread may have been moved from the condition variable queue to the
// mutex queue without a wakeup, by Trans(). In that case, when the thread is
// finally woken, the woken thread will believe it has been woken from the
// condition variable (i.e. its PC will be in when in the CondVar code), when
// in fact it has just been woken from the mutex. Thus, it must enter the slow
// path of the mutex in the same state as if it had just woken from the mutex.
// That is, it must ensure to clear kMuDesig (INV1b).
void Mutex::Trans(MuHow how) {
this->LockSlow(how, nullptr, kMuHasBlocked | kMuIsCond);
}
// Used by CondVar implementation to effectively wake thread w from the
// condition variable. If this mutex is free, we simply wake the thread.
// It will later acquire the mutex with high probability. Otherwise, we
// enqueue thread w on this mutex.
void Mutex::Fer(PerThreadSynch* w) {
SchedulingGuard::ScopedDisable disable_rescheduling;
int c = 0;
ABSL_RAW_CHECK(w->waitp->cond == nullptr,
"Mutex::Fer while waiting on Condition");
ABSL_RAW_CHECK(w->waitp->cv_word == nullptr,
"Mutex::Fer with pending CondVar queueing");
// The CondVar timeout is not relevant for the Mutex wait.
w->waitp->timeout = {};
for (;;) {
intptr_t v = mu_.load(std::memory_order_relaxed);
// Note: must not queue if the mutex is unlocked (nobody will wake it).
// For example, we can have only kMuWait (conditional) or maybe
// kMuWait|kMuWrWait.
// conflicting != 0 implies that the waking thread cannot currently take
// the mutex, which in turn implies that someone else has it and can wake
// us if we queue.
const intptr_t conflicting =
kMuWriter | (w->waitp->how == kShared ? 0 : kMuReader);
if ((v & conflicting) == 0) {
w->next = nullptr;
w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
IncrementSynchSem(this, w);
return;
} else {
if ((v & (kMuSpin | kMuWait)) == 0) { // no waiters
// This thread tries to become the one and only waiter.
PerThreadSynch* new_h =
Enqueue(nullptr, w->waitp, v, kMuIsCond | kMuIsFer);
ABSL_RAW_CHECK(new_h != nullptr,
"Enqueue failed"); // we must queue ourselves
if (mu_.compare_exchange_strong(
v, reinterpret_cast<intptr_t>(new_h) | (v & kMuLow) | kMuWait,
std::memory_order_release, std::memory_order_relaxed)) {
return;
}
} else if ((v & kMuSpin) == 0 &&
mu_.compare_exchange_strong(v, v | kMuSpin | kMuWait)) {
PerThreadSynch* h = GetPerThreadSynch(v);
PerThreadSynch* new_h = Enqueue(h, w->waitp, v, kMuIsCond | kMuIsFer);
ABSL_RAW_CHECK(new_h != nullptr,
"Enqueue failed"); // we must queue ourselves
do {
v = mu_.load(std::memory_order_relaxed);
} while (!mu_.compare_exchange_weak(
v,
(v & kMuLow & ~kMuSpin) | kMuWait |
reinterpret_cast<intptr_t>(new_h),
std::memory_order_release, std::memory_order_relaxed));
return;
}
}
c = synchronization_internal::MutexDelay(c, GENTLE);
}
}
void Mutex::AssertHeld() const {
if ((mu_.load(std::memory_order_relaxed) & kMuWriter) == 0) {
SynchEvent* e = GetSynchEvent(this);
ABSL_RAW_LOG(FATAL, "thread should hold write lock on Mutex %p %s",
static_cast<const void*>(this), (e == nullptr ? "" : e->name));
}
}
void Mutex::AssertReaderHeld() const {
if ((mu_.load(std::memory_order_relaxed) & (kMuReader | kMuWriter)) == 0) {
SynchEvent* e = GetSynchEvent(this);
ABSL_RAW_LOG(FATAL,
"thread should hold at least a read lock on Mutex %p %s",
static_cast<const void*>(this), (e == nullptr ? "" : e->name));
}
}
// -------------------------------- condition variables
static const intptr_t kCvSpin = 0x0001L; // spinlock protects waiter list
static const intptr_t kCvEvent = 0x0002L; // record events
static const intptr_t kCvLow = 0x0003L; // low order bits of CV
// Hack to make constant values available to gdb pretty printer
enum {
kGdbCvSpin = kCvSpin,
kGdbCvEvent = kCvEvent,
kGdbCvLow = kCvLow,
};
static_assert(PerThreadSynch::kAlignment > kCvLow,
"PerThreadSynch::kAlignment must be greater than kCvLow");
void CondVar::EnableDebugLog(const char* name) {
SynchEvent* e = EnsureSynchEvent(&this->cv_, name, kCvEvent, kCvSpin);
e->log = true;
UnrefSynchEvent(e);
}
// Remove thread s from the list of waiters on this condition variable.
void CondVar::Remove(PerThreadSynch* s) {
SchedulingGuard::ScopedDisable disable_rescheduling;
intptr_t v;
int c = 0;
for (v = cv_.load(std::memory_order_relaxed);;
v = cv_.load(std::memory_order_relaxed)) {
if ((v & kCvSpin) == 0 && // attempt to acquire spinlock
cv_.compare_exchange_strong(v, v | kCvSpin, std::memory_order_acquire,
std::memory_order_relaxed)) {
PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
if (h != nullptr) {
PerThreadSynch* w = h;
while (w->next != s && w->next != h) { // search for thread
w = w->next;
}
if (w->next == s) { // found thread; remove it
w->next = s->next;
if (h == s) {
h = (w == s) ? nullptr : w;
}
s->next = nullptr;
s->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
}
}
// release spinlock
cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
std::memory_order_release);
return;
} else {
// try again after a delay
c = synchronization_internal::MutexDelay(c, GENTLE);
}
}
}
// Queue thread waitp->thread on condition variable word cv_word using
// wait parameters waitp.
// We split this into a separate routine, rather than simply doing it as part
// of WaitCommon(). If we were to queue ourselves on the condition variable
// before calling Mutex::UnlockSlow(), the Mutex code might be re-entered (via
// the logging code, or via a Condition function) and might potentially attempt
// to block this thread. That would be a problem if the thread were already on
// a condition variable waiter queue. Thus, we use the waitp->cv_word to tell
// the unlock code to call CondVarEnqueue() to queue the thread on the condition
// variable queue just before the mutex is to be unlocked, and (most
// importantly) after any call to an external routine that might re-enter the
// mutex code.
static void CondVarEnqueue(SynchWaitParams* waitp) {
// This thread might be transferred to the Mutex queue by Fer() when
// we are woken. To make sure that is what happens, Enqueue() doesn't
// call CondVarEnqueue() again but instead uses its normal code. We
// must do this before we queue ourselves so that cv_word will be null
// when seen by the dequeuer, who may wish immediately to requeue
// this thread on another queue.
std::atomic<intptr_t>* cv_word = waitp->cv_word;
waitp->cv_word = nullptr;
intptr_t v = cv_word->load(std::memory_order_relaxed);
int c = 0;
while ((v & kCvSpin) != 0 || // acquire spinlock
!cv_word->compare_exchange_weak(v, v | kCvSpin,
std::memory_order_acquire,
std::memory_order_relaxed)) {
c = synchronization_internal::MutexDelay(c, GENTLE);
v = cv_word->load(std::memory_order_relaxed);
}
ABSL_RAW_CHECK(waitp->thread->waitp == nullptr, "waiting when shouldn't be");
waitp->thread->waitp = waitp; // prepare ourselves for waiting
PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
if (h == nullptr) { // add this thread to waiter list
waitp->thread->next = waitp->thread;
} else {
waitp->thread->next = h->next;
h->next = waitp->thread;
}
waitp->thread->state.store(PerThreadSynch::kQueued,
std::memory_order_relaxed);
cv_word->store((v & kCvEvent) | reinterpret_cast<intptr_t>(waitp->thread),
std::memory_order_release);
}
bool CondVar::WaitCommon(Mutex* mutex, KernelTimeout t) {
bool rc = false; // return value; true iff we timed-out
intptr_t mutex_v = mutex->mu_.load(std::memory_order_relaxed);
Mutex::MuHow mutex_how = ((mutex_v & kMuWriter) != 0) ? kExclusive : kShared;
ABSL_TSAN_MUTEX_PRE_UNLOCK(mutex, TsanFlags(mutex_how));
// maybe trace this call
intptr_t v = cv_.load(std::memory_order_relaxed);
cond_var_tracer("Wait", this);
if ((v & kCvEvent) != 0) {
PostSynchEvent(this, SYNCH_EV_WAIT);
}
// Release mu and wait on condition variable.
SynchWaitParams waitp(mutex_how, nullptr, t, mutex,
Synch_GetPerThreadAnnotated(mutex), &cv_);
// UnlockSlow() will call CondVarEnqueue() just before releasing the
// Mutex, thus queuing this thread on the condition variable. See
// CondVarEnqueue() for the reasons.
mutex->UnlockSlow(&waitp);
// wait for signal
while (waitp.thread->state.load(std::memory_order_acquire) ==
PerThreadSynch::kQueued) {
if (!Mutex::DecrementSynchSem(mutex, waitp.thread, t)) {
// DecrementSynchSem returned due to timeout.
// Now we will either (1) remove ourselves from the wait list in Remove
// below, in which case Remove will set thread.state = kAvailable and
// we will not call DecrementSynchSem again; or (2) Signal/SignalAll
// has removed us concurrently and is calling Wakeup, which will set
// thread.state = kAvailable and post to the semaphore.
// It's important to reset the timeout for the case (2) because otherwise
// we can live-lock in this loop since DecrementSynchSem will always
// return immediately due to timeout, but Signal/SignalAll is not
// necessary set thread.state = kAvailable yet (and is not scheduled
// due to thread priorities or other scheduler artifacts).
// Note this could also be resolved if Signal/SignalAll would set
// thread.state = kAvailable while holding the wait list spin lock.
// But this can't be easily done for SignalAll since it grabs the whole
// wait list with a single compare-exchange and does not really grab
// the spin lock.
t = KernelTimeout::Never();
this->Remove(waitp.thread);
rc = true;
}
}
ABSL_RAW_CHECK(waitp.thread->waitp != nullptr, "not waiting when should be");
waitp.thread->waitp = nullptr; // cleanup
// maybe trace this call
cond_var_tracer("Unwait", this);
if ((v & kCvEvent) != 0) {
PostSynchEvent(this, SYNCH_EV_WAIT_RETURNING);
}
// From synchronization point of view Wait is unlock of the mutex followed
// by lock of the mutex. We've annotated start of unlock in the beginning
// of the function. Now, finish unlock and annotate lock of the mutex.
// (Trans is effectively lock).
ABSL_TSAN_MUTEX_POST_UNLOCK(mutex, TsanFlags(mutex_how));
ABSL_TSAN_MUTEX_PRE_LOCK(mutex, TsanFlags(mutex_how));
mutex->Trans(mutex_how); // Reacquire mutex
ABSL_TSAN_MUTEX_POST_LOCK(mutex, TsanFlags(mutex_how), 0);
return rc;
}
void CondVar::Signal() {
SchedulingGuard::ScopedDisable disable_rescheduling;
ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
intptr_t v;
int c = 0;
for (v = cv_.load(std::memory_order_relaxed); v != 0;
v = cv_.load(std::memory_order_relaxed)) {
if ((v & kCvSpin) == 0 && // attempt to acquire spinlock
cv_.compare_exchange_strong(v, v | kCvSpin, std::memory_order_acquire,
std::memory_order_relaxed)) {
PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
PerThreadSynch* w = nullptr;
if (h != nullptr) { // remove first waiter
w = h->next;
if (w == h) {
h = nullptr;
} else {
h->next = w->next;
}
}
// release spinlock
cv_.store((v & kCvEvent) | reinterpret_cast<intptr_t>(h),
std::memory_order_release);
if (w != nullptr) {
w->waitp->cvmu->Fer(w); // wake waiter, if there was one
cond_var_tracer("Signal wakeup", this);
}
if ((v & kCvEvent) != 0) {
PostSynchEvent(this, SYNCH_EV_SIGNAL);
}
ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
return;
} else {
c = synchronization_internal::MutexDelay(c, GENTLE);
}
}
ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
}
void CondVar::SignalAll() {
ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
intptr_t v;
int c = 0;
for (v = cv_.load(std::memory_order_relaxed); v != 0;
v = cv_.load(std::memory_order_relaxed)) {
// empty the list if spinlock free
// We do this by simply setting the list to empty using
// compare and swap. We then have the entire list in our hands,
// which cannot be changing since we grabbed it while no one
// held the lock.
if ((v & kCvSpin) == 0 &&
cv_.compare_exchange_strong(v, v & kCvEvent, std::memory_order_acquire,
std::memory_order_relaxed)) {
PerThreadSynch* h = reinterpret_cast<PerThreadSynch*>(v & ~kCvLow);
if (h != nullptr) {
PerThreadSynch* w;
PerThreadSynch* n = h->next;
do { // for every thread, wake it up
w = n;
n = n->next;
w->waitp->cvmu->Fer(w);
} while (w != h);
cond_var_tracer("SignalAll wakeup", this);
}
if ((v & kCvEvent) != 0) {
PostSynchEvent(this, SYNCH_EV_SIGNALALL);
}
ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
return;
} else {
// try again after a delay
c = synchronization_internal::MutexDelay(c, GENTLE);
}
}
ABSL_TSAN_MUTEX_POST_SIGNAL(nullptr, 0);
}
void ReleasableMutexLock::Release() {
ABSL_RAW_CHECK(this->mu_ != nullptr,
"ReleasableMutexLock::Release may only be called once");
this->mu_->Unlock();
this->mu_ = nullptr;
}
#ifdef ABSL_HAVE_THREAD_SANITIZER
extern "C" void __tsan_read1(void* addr);
#else
#define __tsan_read1(addr) // do nothing if TSan not enabled
#endif
// A function that just returns its argument, dereferenced
static bool Dereference(void* arg) {
// ThreadSanitizer does not instrument this file for memory accesses.
// This function dereferences a user variable that can participate
// in a data race, so we need to manually tell TSan about this memory access.
__tsan_read1(arg);
return *(static_cast<bool*>(arg));
}
ABSL_CONST_INIT const Condition Condition::kTrue;
Condition::Condition(bool (*func)(void*), void* arg)
: eval_(&CallVoidPtrFunction), arg_(arg) {
static_assert(sizeof(&func) <= sizeof(callback_),
"An overlarge function pointer passed to Condition.");
StoreCallback(func);
}
bool Condition::CallVoidPtrFunction(const Condition* c) {
using FunctionPointer = bool (*)(void*);
FunctionPointer function_pointer;
std::memcpy(&function_pointer, c->callback_, sizeof(function_pointer));
return (*function_pointer)(c->arg_);
}
Condition::Condition(const bool* cond)
: eval_(CallVoidPtrFunction),
// const_cast is safe since Dereference does not modify arg
arg_(const_cast<bool*>(cond)) {
using FunctionPointer = bool (*)(void*);
const FunctionPointer dereference = Dereference;
StoreCallback(dereference);
}
bool Condition::Eval() const { return (*this->eval_)(this); }
bool Condition::GuaranteedEqual(const Condition* a, const Condition* b) {
if (a == nullptr || b == nullptr) {
return a == b;
}
// Check equality of the representative fields.
return a->eval_ == b->eval_ && a->arg_ == b->arg_ &&
!memcmp(a->callback_, b->callback_, sizeof(a->callback_));
}
ABSL_NAMESPACE_END
} // namespace absl