1 // Copyright 2022 The Pigweed Authors 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); you may not 4 // use this file except in compliance with the License. You may obtain a copy of 5 // 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, WITHOUT 11 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 12 // License for the specific language governing permissions and limitations under 13 // the License. 14 #pragma once 15 16 // IMPORTANT: DO NOT USE! 17 // 18 // The interface defined below is currently ONLY implemented by the pw_sync_stl 19 // backed, and cannot be implemented efficiently by other backends. 20 // Specifically, it is difficult for condition variables notifying waiters 21 // efficiently: 22 // 23 // - If wake condition(s) are checked by the waiting threads, these threads must 24 // first be awoken to check the condition, and resume sleeping if it is not 25 // met. These spurious wakeups consume time and power. 26 // - If wake condition(s) are checked by the notifier, synchronization is 27 // required. This prevents the condition variable from being used in ISRs. 28 // Moreover, at least some 29 // 30 // As a result, the interface below WILL either change or be removed. Do not 31 // depend on it. 32 // 33 // TODO(b/294395229): Revisit the interface. 34 35 #include <mutex> 36 37 #include "pw_chrono/system_clock.h" 38 #include "pw_sync/mutex.h" 39 #include "pw_sync_backend/condition_variable_native.h" 40 41 namespace pw::sync { 42 43 // ConditionVariable represents a condition variable using an API very similar 44 // to std::condition_variable. Implementations of this class should share the 45 // same semantics as std::condition_variable. 46 class ConditionVariable { 47 public: 48 using native_handle_type = backend::NativeConditionVariableHandle; 49 50 ConditionVariable() = default; 51 52 ConditionVariable(const ConditionVariable&) = delete; 53 54 ~ConditionVariable() = default; 55 56 ConditionVariable& operator=(const ConditionVariable&) = delete; 57 58 // Wake up one thread waiting on a condition. 59 // 60 // The thread will re-evaluate the condition via its predicate. Threads where 61 // the predicate evaluates false will go back to waiting. The new order of 62 // waiting threads is undefined. 63 void notify_one(); 64 65 // Wake up all threads waiting on the condition variable. 66 // 67 // Woken threads will re-evaluate the condition via their predicate. Threads 68 // where the predicate evaluates false will go back to waiting. The new order 69 // of waiting threads is undefined. 70 void notify_all(); 71 72 // Block the current thread until predicate() == true. 73 // 74 // Precondition: the provided lock must be locked. 75 template <typename Predicate> 76 void wait(std::unique_lock<Mutex>& lock, Predicate predicate); 77 78 // Block the current thread for a duration up to the given timeout or 79 // until predicate() == true whichever comes first. 80 // 81 // Returns: true if predicate() == true. 82 // false if timeout expired. 83 // 84 // Precondition: the provided lock must be locked. 85 template <typename Predicate> 86 bool wait_for(std::unique_lock<Mutex>& lock, 87 pw::chrono::SystemClock::duration timeout, 88 Predicate predicate); 89 90 // Block the current thread until given point in time or until predicate() == 91 // true whichever comes first. 92 // 93 // Returns: true if predicate() == true. 94 // false if the deadline was reached. 95 // 96 // Precondition: the provided lock must be locked. 97 template <typename Predicate> 98 bool wait_until(std::unique_lock<Mutex>& lock, 99 pw::chrono::SystemClock::time_point deadline, 100 Predicate predicate); 101 102 native_handle_type native_handle(); 103 104 private: 105 backend::NativeConditionVariable native_type_; 106 }; 107 108 } // namespace pw::sync 109 110 #include "pw_sync_backend/condition_variable_inline.h" 111