chromium/third_party/abseil-cpp/absl/base/internal/spinlock.h

//
// 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.
//

//  Most users requiring mutual exclusion should use Mutex.
//  SpinLock is provided for use in two situations:
//   - for use by Abseil internal code that Mutex itself depends on
//   - for async signal safety (see below)

// SpinLock with a base_internal::SchedulingMode::SCHEDULE_KERNEL_ONLY is async
// signal safe. If a spinlock is used within a signal handler, all code that
// acquires the lock must ensure that the signal cannot arrive while they are
// holding the lock. Typically, this is done by blocking the signal.
//
// Threads waiting on a SpinLock may be woken in an arbitrary order.

#ifndef ABSL_BASE_INTERNAL_SPINLOCK_H_
#define ABSL_BASE_INTERNAL_SPINLOCK_H_

#include <atomic>
#include <cstdint>

#include "absl/base/attributes.h"
#include "absl/base/const_init.h"
#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/low_level_scheduling.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/scheduling_mode.h"
#include "absl/base/internal/tsan_mutex_interface.h"
#include "absl/base/thread_annotations.h"

namespace tcmalloc {
namespace tcmalloc_internal {

class AllocationGuardSpinLockHolder;

}  // namespace tcmalloc_internal
}  // namespace tcmalloc

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace base_internal {

class ABSL_LOCKABLE ABSL_ATTRIBUTE_WARN_UNUSED SpinLock { … };

// Corresponding locker object that arranges to acquire a spinlock for
// the duration of a C++ scope.
//
// TODO(b/176172494): Use only [[nodiscard]] when baseline is raised.
// TODO(b/6695610): Remove forward declaration when #ifdef is no longer needed.
#if ABSL_HAVE_CPP_ATTRIBUTE(nodiscard)
class [[nodiscard]] SpinLockHolder;
#else
class ABSL_MUST_USE_RESULT ABSL_ATTRIBUTE_TRIVIAL_ABI SpinLockHolder;
#endif

class ABSL_SCOPED_LOCKABLE SpinLockHolder { … };

// Register a hook for profiling support.
//
// The function pointer registered here will be called whenever a spinlock is
// contended.  The callback is given an opaque handle to the contended spinlock
// and the number of wait cycles.  This is thread-safe, but only a single
// profiler can be registered.  It is an error to call this function multiple
// times with different arguments.
void RegisterSpinLockProfiler(void (*fn)(const void* lock,
                                         int64_t wait_cycles));

//------------------------------------------------------------------------------
// Public interface ends here.
//------------------------------------------------------------------------------

// If (result & kSpinLockHeld) == 0, then *this was successfully locked.
// Otherwise, returns last observed value for lockword_.
inline uint32_t SpinLock::TryLockInternal(uint32_t lock_value,
                                          uint32_t wait_cycles) { … }

}  // namespace base_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_SPINLOCK_H_