per_thread_sem.h
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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 
33 
34 namespace absl {
35 
36 class Mutex;
37 
38 namespace synchronization_internal {
39 
40 class PerThreadSem {
41  public:
42  PerThreadSem() = delete;
43  PerThreadSem(const PerThreadSem&) = delete;
44  PerThreadSem& operator=(const PerThreadSem&) = delete;
45 
46  // Routine invoked periodically (once a second) by a background thread.
47  // Has no effect on user-visible state.
48  static void Tick(base_internal::ThreadIdentity* identity);
49 
50  // ---------------------------------------------------------------------------
51  // Routines used by autosizing threadpools to detect when threads are
52  // blocked. Each thread has a counter pointer, initially zero. If non-zero,
53  // the implementation atomically increments the counter when it blocks on a
54  // semaphore, a decrements it again when it wakes. This allows a threadpool
55  // to keep track of how many of its threads are blocked.
56  // SetThreadBlockedCounter() should be used only by threadpool
57  // implementations. GetThreadBlockedCounter() should be used by modules that
58  // block threads; if the pointer returned is non-zero, the location should be
59  // incremented before the thread blocks, and decremented after it wakes.
60  static void SetThreadBlockedCounter(std::atomic<int> *counter);
61  static std::atomic<int> *GetThreadBlockedCounter();
62 
63  private:
64  // Create the PerThreadSem associated with "identity". Initializes count=0.
65  // REQUIRES: May only be called by ThreadIdentity.
66  static void Init(base_internal::ThreadIdentity* identity);
67 
68  // Increments "identity"'s count.
69  static inline void Post(base_internal::ThreadIdentity* identity);
70 
71  // Waits until either our count > 0 or t has expired.
72  // If count > 0, decrements count and returns true. Otherwise returns false.
73  // !t.has_timeout() => Wait(t) will return true.
74  static inline bool Wait(KernelTimeout t);
75 
76  // White-listed callers.
77  friend class PerThreadSemTest;
78  friend class absl::Mutex;
80 };
81 
82 } // namespace synchronization_internal
83 } // namespace absl
84 
85 // In some build configurations we pass --detect-odr-violations to the
86 // gold linker. This causes it to flag weak symbol overrides as ODR
87 // violations. Because ODR only applies to C++ and not C,
88 // --detect-odr-violations ignores symbols not mangled with C++ names.
89 // By changing our extension points to be extern "C", we dodge this
90 // check.
91 extern "C" {
96 } // extern "C"
97 
101 }
102 
106 }
107 
108 #endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
void AbslInternalPerThreadSemPost(absl::base_internal::ThreadIdentity *identity)
static void Init(base_internal::ThreadIdentity *identity)
static void SetThreadBlockedCounter(std::atomic< int > *counter)
static void Post(base_internal::ThreadIdentity *identity)
int * counter
Definition: algorithm.h:29
static void Tick(base_internal::ThreadIdentity *identity)
friend absl::base_internal::ThreadIdentity * CreateThreadIdentity()
static std::atomic< int > * GetThreadBlockedCounter()
bool AbslInternalPerThreadSemWait(absl::synchronization_internal::KernelTimeout t)
PerThreadSem & operator=(const PerThreadSem &)=delete


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autogenerated on Mon Feb 28 2022 21:31:19