synchronization/internal/per_thread_sem.h

0001 // Copyright 2017 The Abseil Authors.
0002 //
0003 // Licensed under the Apache License, Version 2.0 (the "License");
0004 // you may not use this file except in compliance with the License.
0005 // You may obtain a copy of the License at
0006 //
0007 //      https://www.apache.org/licenses/LICENSE-2.0
0008 //
0009 // Unless required by applicable law or agreed to in writing, software
0010 // distributed under the License is distributed on an "AS IS" BASIS,
0011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
0012 // See the License for the specific language governing permissions and
0013 // limitations under the License.
0014 //
0015
0016 // PerThreadSem is a low-level synchronization primitive controlling the
0017 // runnability of a single thread, used internally by Mutex and CondVar.
0018 //
0019 // This is NOT a general-purpose synchronization mechanism, and should not be
0020 // used directly by applications.  Applications should use Mutex and CondVar.
0021 //
0022 // The semantics of PerThreadSem are the same as that of a counting semaphore.
0023 // Each thread maintains an abstract "count" value associated with its identity.
0024
0025 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
0026 #define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
0027
0028 #include <atomic>
0029
0030 #include "absl/base/internal/thread_identity.h"
0031 #include "absl/synchronization/internal/create_thread_identity.h"
0032 #include "absl/synchronization/internal/kernel_timeout.h"
0033
0034 namespace absl {
0035 ABSL_NAMESPACE_BEGIN
0036
0037 class Mutex;
0038
0039 namespace synchronization_internal {
0040
0041 class PerThreadSem {
0042  public:
0043   PerThreadSem() = delete;
0044   PerThreadSem(const PerThreadSem&) = delete;
0045   PerThreadSem& operator=(const PerThreadSem&) = delete;
0046
0047   // Routine invoked periodically (once a second) by a background thread.
0048   // Has no effect on user-visible state.
0049   static void Tick(base_internal::ThreadIdentity* identity);
0050
0051   // ---------------------------------------------------------------------------
0052   // Routines used by autosizing threadpools to detect when threads are
0053   // blocked.  Each thread has a counter pointer, initially zero.  If non-zero,
0054   // the implementation atomically increments the counter when it blocks on a
0055   // semaphore, a decrements it again when it wakes.  This allows a threadpool
0056   // to keep track of how many of its threads are blocked.
0057   // SetThreadBlockedCounter() should be used only by threadpool
0058   // implementations.  GetThreadBlockedCounter() should be used by modules that
0059   // block threads; if the pointer returned is non-zero, the location should be
0060   // incremented before the thread blocks, and decremented after it wakes.
0061   static void SetThreadBlockedCounter(std::atomic<int> *counter);
0062   static std::atomic<int> *GetThreadBlockedCounter();
0063
0064  private:
0065   // Create the PerThreadSem associated with "identity".  Initializes count=0.
0066   // REQUIRES: May only be called by ThreadIdentity.
0067   static inline void Init(base_internal::ThreadIdentity* identity);
0068
0069   // Increments "identity"'s count.
0070   static inline void Post(base_internal::ThreadIdentity* identity);
0071
0072   // Waits until either our count > 0 or t has expired.
0073   // If count > 0, decrements count and returns true.  Otherwise returns false.
0074   // !t.has_timeout() => Wait(t) will return true.
0075   static inline bool Wait(KernelTimeout t);
0076
0077   // Permitted callers.
0078   friend class PerThreadSemTest;
0079   friend class absl::Mutex;
0080   friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*);
0081 };
0082
0083 }  // namespace synchronization_internal
0084 ABSL_NAMESPACE_END
0085 }  // namespace absl
0086
0087 // In some build configurations we pass --detect-odr-violations to the
0088 // gold linker.  This causes it to flag weak symbol overrides as ODR
0089 // violations.  Because ODR only applies to C++ and not C,
0090 // --detect-odr-violations ignores symbols not mangled with C++ names.
0091 // By changing our extension points to be extern "C", we dodge this
0092 // check.
0093 extern "C" {
0094 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(
0095     absl::base_internal::ThreadIdentity* identity);
0096 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(
0097     absl::base_internal::ThreadIdentity* identity);
0098 bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(
0099     absl::synchronization_internal::KernelTimeout t);
0100 void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)(
0101     absl::base_internal::ThreadIdentity* identity);
0102 }  // extern "C"
0103
0104 void absl::synchronization_internal::PerThreadSem::Init(
0105     absl::base_internal::ThreadIdentity* identity) {
0106   ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(identity);
0107 }
0108
0109 void absl::synchronization_internal::PerThreadSem::Post(
0110     absl::base_internal::ThreadIdentity* identity) {
0111   ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(identity);
0112 }
0113
0114 bool absl::synchronization_internal::PerThreadSem::Wait(
0115     absl::synchronization_internal::KernelTimeout t) {
0116   return ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(t);
0117 }
0118
0119 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_