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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/futex_waiter.h"
#ifdef ABSL_INTERNAL_HAVE_FUTEX_WAITER
#include <atomic>
#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"
#include "absl/synchronization/internal/futex.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace synchronization_internal {
#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
constexpr char FutexWaiter::kName[];
#endif
int FutexWaiter::WaitUntil(std::atomic<int32_t>* v, int32_t val,
KernelTimeout t) {
#ifdef CLOCK_MONOTONIC
constexpr bool kHasClockMonotonic = true;
#else
constexpr bool kHasClockMonotonic = false;
#endif
// We can't call Futex::WaitUntil() here because the prodkernel implementation
// does not know about KernelTimeout::SupportsSteadyClock().
if (!t.has_timeout()) {
return Futex::Wait(v, val);
} else if (kHasClockMonotonic && KernelTimeout::SupportsSteadyClock() &&
t.is_relative_timeout()) {
auto rel_timespec = t.MakeRelativeTimespec();
return Futex::WaitRelativeTimeout(v, val, &rel_timespec);
} else {
auto abs_timespec = t.MakeAbsTimespec();
return Futex::WaitAbsoluteTimeout(v, val, &abs_timespec);
}
}
bool FutexWaiter::Wait(KernelTimeout t) {
// Loop until we can atomically decrement futex from a positive
// value, waiting on a futex while we believe it is zero.
// 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) {
int32_t x = futex_.load(std::memory_order_relaxed);
while (x != 0) {
if (!futex_.compare_exchange_weak(x, x - 1,
std::memory_order_acquire,
std::memory_order_relaxed)) {
continue; // Raced with someone, retry.
}
return true; // Consumed a wakeup, we are done.
}
if (!first_pass) MaybeBecomeIdle();
const int err = WaitUntil(&futex_, 0, t);
if (err != 0) {
if (err == -EINTR || err == -EWOULDBLOCK) {
// Do nothing, the loop will retry.
} else if (err == -ETIMEDOUT) {
return false;
} else {
ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
}
}
first_pass = false;
}
}
void FutexWaiter::Post() {
if (futex_.fetch_add(1, std::memory_order_release) == 0) {
// We incremented from 0, need to wake a potential waiter.
Poke();
}
}
void FutexWaiter::Poke() {
// Wake one thread waiting on the futex.
const int err = Futex::Wake(&futex_, 1);
if (ABSL_PREDICT_FALSE(err < 0)) {
ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
}
}
} // namespace synchronization_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_INTERNAL_HAVE_FUTEX_WAITER
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