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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.
//
#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
#include "absl/base/config.h"
#ifdef _WIN32
#include <sdkddkver.h>
#else
#include <pthread.h>
#endif
#ifdef __linux__
#include <linux/futex.h>
#endif
#ifdef ABSL_HAVE_SEMAPHORE_H
#include <semaphore.h>
#endif
#include <atomic>
#include <cstdint>
#include "absl/base/internal/thread_identity.h"
#include "absl/synchronization/internal/futex.h"
#include "absl/synchronization/internal/kernel_timeout.h"
// May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
#define ABSL_WAITER_MODE_FUTEX 0
#define ABSL_WAITER_MODE_SEM 1
#define ABSL_WAITER_MODE_CONDVAR 2
#define ABSL_WAITER_MODE_WIN32 3
#if defined(ABSL_FORCE_WAITER_MODE)
#define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
#elif defined(ABSL_INTERNAL_HAVE_FUTEX)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
#elif defined(ABSL_HAVE_SEMAPHORE_H)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
#else
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
#endif
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace synchronization_internal {
// Waiter is an OS-specific semaphore.
class Waiter {
public:
// Prepare any data to track waits.
Waiter();
// Not copyable or movable
Waiter(const Waiter&) = delete;
Waiter& operator=(const Waiter&) = delete;
// Blocks the calling thread until a matching call to `Post()` or
// `t` has passed. Returns `true` if woken (`Post()` called),
// `false` on timeout.
bool Wait(KernelTimeout t);
// Restart the caller of `Wait()` as with a normal semaphore.
void Post();
// If anyone is waiting, wake them up temporarily and cause them to
// call `MaybeBecomeIdle()`. They will then return to waiting for a
// `Post()` or timeout.
void Poke();
// Returns the Waiter associated with the identity.
static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
static_assert(
sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
"Insufficient space for Waiter");
return reinterpret_cast<Waiter*>(identity->waiter_state.data);
}
// How many periods to remain idle before releasing resources
#ifndef ABSL_HAVE_THREAD_SANITIZER
static constexpr int kIdlePeriods = 60;
#else
// Memory consumption under ThreadSanitizer is a serious concern,
// so we release resources sooner. The value of 1 leads to 1 to 2 second
// delay before marking a thread as idle.
static const int kIdlePeriods = 1;
#endif
private:
// The destructor must not be called since Mutex/CondVar
// can use PerThreadSem/Waiter after the thread exits.
// Waiter objects are embedded in ThreadIdentity objects,
// which are reused via a freelist and are never destroyed.
~Waiter() = delete;
#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
// Futexes are defined by specification to be 32-bits.
// Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
std::atomic<int32_t> futex_;
static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
// REQUIRES: mu_ must be held.
void InternalCondVarPoke();
pthread_mutex_t mu_;
pthread_cond_t cv_;
int waiter_count_;
int wakeup_count_; // Unclaimed wakeups.
#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
sem_t sem_;
// This seems superfluous, but for Poke() we need to cause spurious
// wakeups on the semaphore. Hence we can't actually use the
// semaphore's count.
std::atomic<int> wakeups_;
#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
// WinHelper - Used to define utilities for accessing the lock and
// condition variable storage once the types are complete.
class WinHelper;
// REQUIRES: WinHelper::GetLock(this) must be held.
void InternalCondVarPoke();
// We can't include Windows.h in our headers, so we use aligned character
// buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
// SRW locks and condition variables do not need to be explicitly destroyed.
// https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
// https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
alignas(void*) unsigned char mu_storage_[sizeof(void*)];
alignas(void*) unsigned char cv_storage_[sizeof(void*)];
int waiter_count_;
int wakeup_count_;
#else
#error Unknown ABSL_WAITER_MODE
#endif
};
} // namespace synchronization_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
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