barrier.h (2866B)
1 // Copyright 2017 The Abseil Authors. 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // https://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 // 15 // ----------------------------------------------------------------------------- 16 // barrier.h 17 // ----------------------------------------------------------------------------- 18 19 #ifndef ABSL_SYNCHRONIZATION_BARRIER_H_ 20 #define ABSL_SYNCHRONIZATION_BARRIER_H_ 21 22 #include "absl/base/thread_annotations.h" 23 #include "absl/synchronization/mutex.h" 24 25 namespace absl { 26 ABSL_NAMESPACE_BEGIN 27 28 // Barrier 29 // 30 // This class creates a barrier which blocks threads until a prespecified 31 // threshold of threads (`num_threads`) utilizes the barrier. A thread utilizes 32 // the `Barrier` by calling `Block()` on the barrier, which will block that 33 // thread; no call to `Block()` will return until `num_threads` threads have 34 // called it. 35 // 36 // Exactly one call to `Block()` will return `true`, which is then responsible 37 // for destroying the barrier; because stack allocation will cause the barrier 38 // to be deleted when it is out of scope, barriers should not be stack 39 // allocated. 40 // 41 // Example: 42 // 43 // // Main thread creates a `Barrier`: 44 // barrier = new Barrier(num_threads); 45 // 46 // // Each participating thread could then call: 47 // if (barrier->Block()) delete barrier; // Exactly one call to `Block()` 48 // // returns `true`; that call 49 // // deletes the barrier. 50 class Barrier { 51 public: 52 // `num_threads` is the number of threads that will participate in the barrier 53 explicit Barrier(int num_threads) 54 : num_to_block_(num_threads), num_to_exit_(num_threads) {} 55 56 Barrier(const Barrier&) = delete; 57 Barrier& operator=(const Barrier&) = delete; 58 59 // Barrier::Block() 60 // 61 // Blocks the current thread, and returns only when the `num_threads` 62 // threshold of threads utilizing this barrier has been reached. `Block()` 63 // returns `true` for precisely one caller, which may then destroy the 64 // barrier. 65 // 66 // Memory ordering: For any threads X and Y, any action taken by X 67 // before X calls `Block()` will be visible to Y after Y returns from 68 // `Block()`. 69 bool Block(); 70 71 private: 72 Mutex lock_; 73 int num_to_block_ ABSL_GUARDED_BY(lock_); 74 int num_to_exit_ ABSL_GUARDED_BY(lock_); 75 }; 76 77 ABSL_NAMESPACE_END 78 } // namespace absl 79 #endif // ABSL_SYNCHRONIZATION_BARRIER_H_