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 {
35
36class Mutex;
37
38namespace synchronization_internal {
39
40class 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;
79 friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
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.
91extern "C" {
92void AbslInternalPerThreadSemPost(
93 absl::base_internal::ThreadIdentity* identity);
94bool AbslInternalPerThreadSemWait(
95 absl::synchronization_internal::KernelTimeout t);
96} // extern "C"
97
98void absl::synchronization_internal::PerThreadSem::Post(
99 absl::base_internal::ThreadIdentity* identity) {
100 AbslInternalPerThreadSemPost(identity);
101}
102
103bool absl::synchronization_internal::PerThreadSem::Wait(
104 absl::synchronization_internal::KernelTimeout t) {
105 return AbslInternalPerThreadSemWait(t);
106}
107
108#endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
109