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 void Init(base_internal::ThreadIdentity* identity);
68
69 // Destroy the PerThreadSem associated with "identity".
70 // REQUIRES: May only be called by ThreadIdentity.
71 static void Destroy(base_internal::ThreadIdentity* identity);
72
73 // Increments "identity"'s count.
74 static inline void Post(base_internal::ThreadIdentity* identity);
75
76 // Waits until either our count > 0 or t has expired.
77 // If count > 0, decrements count and returns true. Otherwise returns false.
78 // !t.has_timeout() => Wait(t) will return true.
79 static inline bool Wait(KernelTimeout t);
80
81 // Permitted callers.
82 friend class PerThreadSemTest;
83 friend class absl::Mutex;
84 friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
85 friend void ReclaimThreadIdentity(void* v);
86 };
87
88 } // namespace synchronization_internal
89 ABSL_NAMESPACE_END
90 } // namespace absl
91
92 // In some build configurations we pass --detect-odr-violations to the
93 // gold linker. This causes it to flag weak symbol overrides as ODR
94 // violations. Because ODR only applies to C++ and not C,
95 // --detect-odr-violations ignores symbols not mangled with C++ names.
96 // By changing our extension points to be extern "C", we dodge this
97 // check.
98 extern "C" {
99 void AbslInternalPerThreadSemPost(
100 absl::base_internal::ThreadIdentity* identity);
101 bool AbslInternalPerThreadSemWait(
102 absl::synchronization_internal::KernelTimeout t);
103 } // extern "C"
104
Post(absl::base_internal::ThreadIdentity * identity)105 void absl::synchronization_internal::PerThreadSem::Post(
106 absl::base_internal::ThreadIdentity* identity) {
107 AbslInternalPerThreadSemPost(identity);
108 }
109
Wait(absl::synchronization_internal::KernelTimeout t)110 bool absl::synchronization_internal::PerThreadSem::Wait(
111 absl::synchronization_internal::KernelTimeout t) {
112 return AbslInternalPerThreadSemWait(t);
113 }
114
115 #endif // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
116