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
34namespace absl {
35ABSL_NAMESPACE_BEGIN
36
37class Mutex;
38
39namespace synchronization_internal {
40
41class 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 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
84ABSL_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.
93extern "C" {
94void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(
95 absl::base_internal::ThreadIdentity* identity);
96bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(
97 absl::synchronization_internal::KernelTimeout t);
98} // extern "C"
99
100void absl::synchronization_internal::PerThreadSem::Post(
101 absl::base_internal::ThreadIdentity* identity) {
102 ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(identity);
103}
104
105bool absl::synchronization_internal::PerThreadSem::Wait(
106 absl::synchronization_internal::KernelTimeout t) {
107 return ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(t);
108}
109
110#endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
111

source code of include/absl/synchronization/internal/per_thread_sem.h