per_thread_sem.h (4897B)
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 // PerThreadSem is a low-level synchronization primitive controlling the 17 // runnability of a single thread, used internally by Mutex and CondVar. 18 // 19 // This is NOT a general-purpose synchronization mechanism, and should not be 20 // used directly by applications. Applications should use Mutex and CondVar. 21 // 22 // The semantics of PerThreadSem are the same as that of a counting semaphore. 23 // Each thread maintains an abstract "count" value associated with its identity. 24 25 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_ 26 #define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_ 27 28 #include <atomic> 29 30 #include "absl/base/internal/thread_identity.h" 31 #include "absl/synchronization/internal/create_thread_identity.h" 32 #include "absl/synchronization/internal/kernel_timeout.h" 33 34 namespace absl { 35 ABSL_NAMESPACE_BEGIN 36 37 class Mutex; 38 39 namespace synchronization_internal { 40 41 class PerThreadSem { 42 public: 43 PerThreadSem() = delete; 44 PerThreadSem(const PerThreadSem&) = delete; 45 PerThreadSem& operator=(const PerThreadSem&) = delete; 46 47 // Routine invoked periodically (once a second) by a background thread. 48 // Has no effect on user-visible state. 49 static void Tick(base_internal::ThreadIdentity* identity); 50 51 // --------------------------------------------------------------------------- 52 // Routines used by autosizing threadpools to detect when threads are 53 // blocked. Each thread has a counter pointer, initially zero. If non-zero, 54 // the implementation atomically increments the counter when it blocks on a 55 // semaphore, a decrements it again when it wakes. This allows a threadpool 56 // to keep track of how many of its threads are blocked. 57 // SetThreadBlockedCounter() should be used only by threadpool 58 // implementations. GetThreadBlockedCounter() should be used by modules that 59 // block threads; if the pointer returned is non-zero, the location should be 60 // incremented before the thread blocks, and decremented after it wakes. 61 static void SetThreadBlockedCounter(std::atomic<int> *counter); 62 static std::atomic<int> *GetThreadBlockedCounter(); 63 64 private: 65 // Create the PerThreadSem associated with "identity". Initializes count=0. 66 // REQUIRES: May only be called by ThreadIdentity. 67 static inline void Init(base_internal::ThreadIdentity* identity); 68 69 // Increments "identity"'s count. 70 static inline void Post(base_internal::ThreadIdentity* identity); 71 72 // Waits until either our count > 0 or t has expired. 73 // If count > 0, decrements count and returns true. Otherwise returns false. 74 // !t.has_timeout() => Wait(t) will return true. 75 static inline bool Wait(KernelTimeout t); 76 77 // Permitted callers. 78 friend class PerThreadSemTest; 79 friend class absl::Mutex; 80 friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*); 81 }; 82 83 } // namespace synchronization_internal 84 ABSL_NAMESPACE_END 85 } // namespace absl 86 87 // In some build configurations we pass --detect-odr-violations to the 88 // gold linker. This causes it to flag weak symbol overrides as ODR 89 // violations. Because ODR only applies to C++ and not C, 90 // --detect-odr-violations ignores symbols not mangled with C++ names. 91 // By changing our extension points to be extern "C", we dodge this 92 // check. 93 extern "C" { 94 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)( 95 absl::base_internal::ThreadIdentity* identity); 96 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)( 97 absl::base_internal::ThreadIdentity* identity); 98 bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)( 99 absl::synchronization_internal::KernelTimeout t); 100 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)( 101 absl::base_internal::ThreadIdentity* identity); 102 } // extern "C" 103 104 void absl::synchronization_internal::PerThreadSem::Init( 105 absl::base_internal::ThreadIdentity* identity) { 106 ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(identity); 107 } 108 109 void absl::synchronization_internal::PerThreadSem::Post( 110 absl::base_internal::ThreadIdentity* identity) { 111 ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(identity); 112 } 113 114 bool absl::synchronization_internal::PerThreadSem::Wait( 115 absl::synchronization_internal::KernelTimeout t) { 116 return ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(t); 117 } 118 119 #endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_