waiter.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 #ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
17 #define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
18 
19 #include "absl/base/config.h"
20 
21 #ifndef _WIN32
22 #include <pthread.h>
23 #endif
24 
25 #ifdef ABSL_HAVE_SEMAPHORE_H
26 #include <semaphore.h>
27 #endif
28 
29 #include <atomic>
30 #include <cstdint>
31 
34 
35 // May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
36 #define ABSL_WAITER_MODE_FUTEX 0
37 #define ABSL_WAITER_MODE_SEM 1
38 #define ABSL_WAITER_MODE_CONDVAR 2
39 #define ABSL_WAITER_MODE_WIN32 3
40 
41 #if defined(ABSL_FORCE_WAITER_MODE)
42 #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
43 #elif defined(_WIN32)
44 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
45 #elif defined(__linux__)
46 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
47 #elif defined(ABSL_HAVE_SEMAPHORE_H)
48 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
49 #else
50 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
51 #endif
52 
53 namespace absl {
54 namespace synchronization_internal {
55 
56 // Waiter is an OS-specific semaphore.
57 class Waiter {
58  public:
59  // No constructor, instances use the reserved space in ThreadIdentity.
60  // All initialization logic belongs in `Init()`.
61  Waiter() = delete;
62  Waiter(const Waiter&) = delete;
63  Waiter& operator=(const Waiter&) = delete;
64 
65  // Prepare any data to track waits.
66  void Init();
67 
68  // Blocks the calling thread until a matching call to `Post()` or
69  // `t` has passed. Returns `true` if woken (`Post()` called),
70  // `false` on timeout.
71  bool Wait(KernelTimeout t);
72 
73  // Restart the caller of `Wait()` as with a normal semaphore.
74  void Post();
75 
76  // If anyone is waiting, wake them up temporarily and cause them to
77  // call `MaybeBecomeIdle()`. They will then return to waiting for a
78  // `Post()` or timeout.
79  void Poke();
80 
81  // Returns the Waiter associated with the identity.
83  static_assert(
85  "Insufficient space for Waiter");
86  return reinterpret_cast<Waiter*>(identity->waiter_state.data);
87  }
88 
89  // How many periods to remain idle before releasing resources
90 #ifndef THREAD_SANITIZER
91  static const int kIdlePeriods = 60;
92 #else
93  // Memory consumption under ThreadSanitizer is a serious concern,
94  // so we release resources sooner. The value of 1 leads to 1 to 2 second
95  // delay before marking a thread as idle.
96  static const int kIdlePeriods = 1;
97 #endif
98 
99  private:
100 #if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
101  // Futexes are defined by specification to be 32-bits.
102  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
103  std::atomic<int32_t> futex_;
104  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
105 
106 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
107  pthread_mutex_t mu_;
108  pthread_cond_t cv_;
109  std::atomic<int> waiter_count_;
110  std::atomic<int> wakeup_count_; // Unclaimed wakeups, written under lock.
111 
112 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
113  sem_t sem_;
114  // This seems superfluous, but for Poke() we need to cause spurious
115  // wakeups on the semaphore. Hence we can't actually use the
116  // semaphore's count.
117  std::atomic<int> wakeups_;
118 
119 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
120  // The Windows API has lots of choices for synchronization
121  // primivitives. We are using SRWLOCK and CONDITION_VARIABLE
122  // because they don't require a destructor to release system
123  // resources.
124  //
125  // However, we can't include Windows.h in our headers, so we use aligned
126  // storage buffers to define the storage.
127  using SRWLockStorage =
128  typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
129  using ConditionVariableStorage =
130  typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
131 
132  // WinHelper - Used to define utilities for accessing the lock and
133  // condition variable storage once the types are complete.
134  class WinHelper;
135 
136  SRWLockStorage mu_storage_;
137  ConditionVariableStorage cv_storage_;
138  std::atomic<int> waiter_count_;
139  std::atomic<int> wakeup_count_;
140 
141 #else
142  #error Unknown ABSL_WAITER_MODE
143 #endif
144 };
145 
146 } // namespace synchronization_internal
147 } // namespace absl
148 
149 #endif // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
bool Wait(KernelTimeout t)
Definition: waiter.cc:129
std::atomic< int > wakeup_count_
Definition: waiter.h:110
Waiter & operator=(const Waiter &)=delete
std::atomic< int > waiter_count_
Definition: waiter.h:109
Definition: algorithm.h:29
struct absl::base_internal::ThreadIdentity::WaiterState waiter_state
static Waiter * GetWaiter(base_internal::ThreadIdentity *identity)
Definition: waiter.h:82


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autogenerated on Mon Feb 28 2022 21:31:21