fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
99 lines
3.7 KiB
C++
99 lines
3.7 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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// 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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// blocking_counter.h
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// -----------------------------------------------------------------------------
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#ifndef ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
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#define ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_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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// BlockingCounter
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//
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// This class allows a thread to block for a pre-specified number of actions.
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// `BlockingCounter` maintains a single non-negative abstract integer "count"
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// with an initial value `initial_count`. A thread can then call `Wait()` on
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// this blocking counter to block until the specified number of events occur;
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// worker threads then call 'DecrementCount()` on the counter upon completion of
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// their work. Once the counter's internal "count" reaches zero, the blocked
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// thread unblocks.
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//
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// A `BlockingCounter` requires the following:
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// - its `initial_count` is non-negative.
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// - the number of calls to `DecrementCount()` on it is at most
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// `initial_count`.
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// - `Wait()` is called at most once on it.
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//
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// Given the above requirements, a `BlockingCounter` provides the following
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// guarantees:
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// - Once its internal "count" reaches zero, no legal action on the object
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// can further change the value of "count".
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// - When `Wait()` returns, it is legal to destroy the `BlockingCounter`.
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// - When `Wait()` returns, the number of calls to `DecrementCount()` on
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// this blocking counter exactly equals `initial_count`.
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//
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// Example:
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// BlockingCounter bcount(N); // there are N items of work
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// ... Allow worker threads to start.
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// ... On completing each work item, workers do:
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// ... bcount.DecrementCount(); // an item of work has been completed
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//
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// bcount.Wait(); // wait for all work to be complete
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//
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class BlockingCounter {
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public:
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explicit BlockingCounter(int initial_count)
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: count_(initial_count), num_waiting_(0) {}
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BlockingCounter(const BlockingCounter&) = delete;
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BlockingCounter& operator=(const BlockingCounter&) = delete;
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// BlockingCounter::DecrementCount()
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//
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// Decrements the counter's "count" by one, and return "count == 0". This
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// function requires that "count != 0" when it is called.
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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 it calls `DecrementCount()` is visible to thread Y after
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// Y's call to `DecrementCount()`, provided Y's call returns `true`.
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bool DecrementCount();
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// BlockingCounter::Wait()
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//
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// Blocks until the counter reaches zero. This function may be called at most
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// once. On return, `DecrementCount()` will have been called "initial_count"
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// times and the blocking counter may be destroyed.
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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 `DecrementCount()` is visible to Y after Y returns
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// from `Wait()`.
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void Wait();
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private:
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Mutex lock_;
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int count_ ABSL_GUARDED_BY(lock_);
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int num_waiting_ 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_BLOCKING_COUNTER_H_
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