108 lines
4.2 KiB
C
108 lines
4.2 KiB
C
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// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// PerThreadSem is a low-level synchronization primitive controlling the
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// runnability of a single thread, used internally by Mutex and CondVar.
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//
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// This is NOT a general-purpose synchronization mechanism, and should not be
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// used directly by applications. Applications should use Mutex and CondVar.
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//
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// The semantics of PerThreadSem are the same as that of a counting semaphore.
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// Each thread maintains an abstract "count" value associated with its identity.
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#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
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#define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
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#include <atomic>
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#include "absl/base/internal/thread_identity.h"
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#include "absl/synchronization/internal/create_thread_identity.h"
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#include "absl/synchronization/internal/kernel_timeout.h"
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namespace absl {
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class Mutex;
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namespace synchronization_internal {
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class PerThreadSem {
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public:
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PerThreadSem() = delete;
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PerThreadSem(const PerThreadSem&) = delete;
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PerThreadSem& operator=(const PerThreadSem&) = delete;
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// Routine invoked periodically (once a second) by a background thread.
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// Has no effect on user-visible state.
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static void Tick(base_internal::ThreadIdentity* identity);
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// ---------------------------------------------------------------------------
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// Routines used by autosizing threadpools to detect when threads are
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// blocked. Each thread has a counter pointer, initially zero. If non-zero,
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// the implementation atomically increments the counter when it blocks on a
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// semaphore, a decrements it again when it wakes. This allows a threadpool
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// to keep track of how many of its threads are blocked.
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// SetThreadBlockedCounter() should be used only by threadpool
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// implementations. GetThreadBlockedCounter() should be used by modules that
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// block threads; if the pointer returned is non-zero, the location should be
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// incremented before the thread blocks, and decremented after it wakes.
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static void SetThreadBlockedCounter(std::atomic<int> *counter);
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static std::atomic<int> *GetThreadBlockedCounter();
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private:
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// Create the PerThreadSem associated with "identity". Initializes count=0.
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// REQUIRES: May only be called by ThreadIdentity.
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static void Init(base_internal::ThreadIdentity* identity);
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// Increments "identity"'s count.
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static inline void Post(base_internal::ThreadIdentity* identity);
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// Waits until either our count > 0 or t has expired.
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// If count > 0, decrements count and returns true. Otherwise returns false.
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// !t.has_timeout() => Wait(t) will return true.
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static inline bool Wait(KernelTimeout t);
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// White-listed callers.
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friend class PerThreadSemTest;
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friend class absl::Mutex;
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friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
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};
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} // namespace synchronization_internal
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} // namespace absl
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// In some build configurations we pass --detect-odr-violations to the
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// gold linker. This causes it to flag weak symbol overrides as ODR
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// violations. Because ODR only applies to C++ and not C,
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// --detect-odr-violations ignores symbols not mangled with C++ names.
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// By changing our extension points to be extern "C", we dodge this
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// check.
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extern "C" {
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void AbslInternalPerThreadSemPost(
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absl::base_internal::ThreadIdentity* identity);
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bool AbslInternalPerThreadSemWait(
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absl::synchronization_internal::KernelTimeout t);
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} // extern "C"
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void absl::synchronization_internal::PerThreadSem::Post(
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absl::base_internal::ThreadIdentity* identity) {
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AbslInternalPerThreadSemPost(identity);
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}
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bool absl::synchronization_internal::PerThreadSem::Wait(
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absl::synchronization_internal::KernelTimeout t) {
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return AbslInternalPerThreadSemWait(t);
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}
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#endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
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