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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
//
// http://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.
//
// PerThreadSem is a low-level synchronization primitive controlling the
// runnability of a single thread, used internally by Mutex and CondVar.
//
// This is NOT a general-purpose synchronization mechanism, and should not be
// used directly by applications. Applications should use Mutex and CondVar.
//
// The semantics of PerThreadSem are the same as that of a counting semaphore.
// Each thread maintains an abstract "count" value associated with its identity.
#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
#define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
#include <atomic>
#include "absl/base/internal/thread_identity.h"
#include "absl/synchronization/internal/create_thread_identity.h"
#include "absl/synchronization/internal/kernel_timeout.h"
namespace absl {
inline namespace lts_2018_06_20 {
class Mutex;
namespace synchronization_internal {
class PerThreadSem {
public:
PerThreadSem() = delete;
PerThreadSem(const PerThreadSem&) = delete;
PerThreadSem& operator=(const PerThreadSem&) = delete;
// Routine invoked periodically (once a second) by a background thread.
// Has no effect on user-visible state.
static void Tick(base_internal::ThreadIdentity* identity);
// ---------------------------------------------------------------------------
// Routines used by autosizing threadpools to detect when threads are
// blocked. Each thread has a counter pointer, initially zero. If non-zero,
// the implementation atomically increments the counter when it blocks on a
// semaphore, a decrements it again when it wakes. This allows a threadpool
// to keep track of how many of its threads are blocked.
// SetThreadBlockedCounter() should be used only by threadpool
// implementations. GetThreadBlockedCounter() should be used by modules that
// block threads; if the pointer returned is non-zero, the location should be
// incremented before the thread blocks, and decremented after it wakes.
static void SetThreadBlockedCounter(std::atomic<int> *counter);
static std::atomic<int> *GetThreadBlockedCounter();
private:
// Create the PerThreadSem associated with "identity". Initializes count=0.
// REQUIRES: May only be called by ThreadIdentity.
static void Init(base_internal::ThreadIdentity* identity);
// Increments "identity"'s count.
static inline void Post(base_internal::ThreadIdentity* identity);
// Waits until either our count > 0 or t has expired.
// If count > 0, decrements count and returns true. Otherwise returns false.
// !t.has_timeout() => Wait(t) will return true.
static inline bool Wait(KernelTimeout t);
// White-listed callers.
friend class PerThreadSemTest;
friend class absl::Mutex;
friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
};
} // namespace synchronization_internal
} // inline namespace lts_2018_06_20
} // namespace absl
// In some build configurations we pass --detect-odr-violations to the
// gold linker. This causes it to flag weak symbol overrides as ODR
// violations. Because ODR only applies to C++ and not C,
// --detect-odr-violations ignores symbols not mangled with C++ names.
// By changing our extension points to be extern "C", we dodge this
// check.
extern "C" {
void AbslInternalPerThreadSemPost(
absl::base_internal::ThreadIdentity* identity);
bool AbslInternalPerThreadSemWait(
absl::synchronization_internal::KernelTimeout t);
} // extern "C"
void absl::synchronization_internal::PerThreadSem::Post(
absl::base_internal::ThreadIdentity* identity) {
AbslInternalPerThreadSemPost(identity);
}
bool absl::synchronization_internal::PerThreadSem::Wait(
absl::synchronization_internal::KernelTimeout t) {
return AbslInternalPerThreadSemWait(t);
}
#endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
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