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// Copyright 2023 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/internal/sem_waiter.h"
#ifdef ABSL_INTERNAL_HAVE_SEM_WAITER
#include <semaphore.h>
#include <atomic>
#include <cassert>
#include <cstdint>
#include <cerrno>
#include "absl/base/config.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/thread_identity.h"
#include "absl/base/optimization.h"
#include "absl/synchronization/internal/kernel_timeout.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace synchronization_internal {
#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
constexpr char SemWaiter::kName[];
#endif
SemWaiter::SemWaiter() : wakeups_(0) {
if (sem_init(&sem_, 0, 0) != 0) {
ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
}
}
bool SemWaiter::Wait(KernelTimeout t) {
struct timespec abs_timeout;
if (t.has_timeout()) {
abs_timeout = t.MakeAbsTimespec();
}
// Loop until we timeout or consume a wakeup.
// Note that, since the thread ticker is just reset, we don't need to check
// whether the thread is idle on the very first pass of the loop.
bool first_pass = true;
while (true) {
int x = wakeups_.load(std::memory_order_relaxed);
while (x != 0) {
if (!wakeups_.compare_exchange_weak(x, x - 1,
std::memory_order_acquire,
std::memory_order_relaxed)) {
continue; // Raced with someone, retry.
}
// Successfully consumed a wakeup, we're done.
return true;
}
if (!first_pass) MaybeBecomeIdle();
// Nothing to consume, wait (looping on EINTR).
while (true) {
if (!t.has_timeout()) {
if (sem_wait(&sem_) == 0) break;
if (errno == EINTR) continue;
ABSL_RAW_LOG(FATAL, "sem_wait failed: %d", errno);
} else {
if (sem_timedwait(&sem_, &abs_timeout) == 0) break;
if (errno == EINTR) continue;
if (errno == ETIMEDOUT) return false;
ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
}
}
first_pass = false;
}
}
void SemWaiter::Post() {
// Post a wakeup.
if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
// We incremented from 0, need to wake a potential waiter.
Poke();
}
}
void SemWaiter::Poke() {
if (sem_post(&sem_) != 0) { // Wake any semaphore waiter.
ABSL_RAW_LOG(FATAL, "sem_post failed with errno %d\n", errno);
}
}
} // namespace synchronization_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_INTERNAL_HAVE_SEM_WAITER
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