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80 lines
2.8 KiB
80 lines
2.8 KiB
4 months ago
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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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// https://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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// -----------------------------------------------------------------------------
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// barrier.h
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// -----------------------------------------------------------------------------
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#ifndef ABSL_SYNCHRONIZATION_BARRIER_H_
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#define ABSL_SYNCHRONIZATION_BARRIER_H_
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#include "absl/base/thread_annotations.h"
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#include "absl/synchronization/mutex.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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// Barrier
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//
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// This class creates a barrier which blocks threads until a prespecified
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// threshold of threads (`num_threads`) utilizes the barrier. A thread utilizes
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// the `Barrier` by calling `Block()` on the barrier, which will block that
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// thread; no call to `Block()` will return until `num_threads` threads have
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// called it.
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//
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// Exactly one call to `Block()` will return `true`, which is then responsible
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// for destroying the barrier; because stack allocation will cause the barrier
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// to be deleted when it is out of scope, barriers should not be stack
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// allocated.
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//
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// Example:
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//
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// // Main thread creates a `Barrier`:
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// barrier = new Barrier(num_threads);
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//
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// // Each participating thread could then call:
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// if (barrier->Block()) delete barrier; // Exactly one call to `Block()`
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// // returns `true`; that call
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// // deletes the barrier.
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class Barrier {
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public:
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// `num_threads` is the number of threads that will participate in the barrier
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explicit Barrier(int num_threads)
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: num_to_block_(num_threads), num_to_exit_(num_threads) {}
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Barrier(const Barrier&) = delete;
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Barrier& operator=(const Barrier&) = delete;
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// Barrier::Block()
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//
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// Blocks the current thread, and returns only when the `num_threads`
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// threshold of threads utilizing this barrier has been reached. `Block()`
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// returns `true` for precisely one caller, which may then destroy the
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// barrier.
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//
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// Memory ordering: For any threads X and Y, any action taken by X
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// before X calls `Block()` will be visible to Y after Y returns from
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// `Block()`.
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bool Block();
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private:
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Mutex lock_;
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int num_to_block_ ABSL_GUARDED_BY(lock_);
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int num_to_exit_ ABSL_GUARDED_BY(lock_);
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};
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_SYNCHRONIZATION_BARRIER_H_
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