summaryrefslogtreecommitdiff
path: root/absl/synchronization/internal/waiter.h
blob: be3df180d4e26725bd2f2780acd19a8ec711a869 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_

#include "absl/base/config.h"

#ifdef _WIN32
#include <sdkddkver.h>
#else
#include <pthread.h>
#endif

#ifdef __linux__
#include <linux/futex.h>
#endif

#ifdef ABSL_HAVE_SEMAPHORE_H
#include <semaphore.h>
#endif

#include <atomic>
#include <cstdint>

#include "absl/base/internal/thread_identity.h"
#include "absl/synchronization/internal/futex.h"
#include "absl/synchronization/internal/kernel_timeout.h"

// May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
#define ABSL_WAITER_MODE_FUTEX 0
#define ABSL_WAITER_MODE_SEM 1
#define ABSL_WAITER_MODE_CONDVAR 2
#define ABSL_WAITER_MODE_WIN32 3

#if defined(ABSL_FORCE_WAITER_MODE)
#define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
#elif defined(ABSL_INTERNAL_HAVE_FUTEX)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
#elif defined(ABSL_HAVE_SEMAPHORE_H)
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
#else
#define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace synchronization_internal {

// Waiter is an OS-specific semaphore.
class Waiter {
 public:
  // Prepare any data to track waits.
  Waiter();

  // Not copyable or movable
  Waiter(const Waiter&) = delete;
  Waiter& operator=(const Waiter&) = delete;

  // Destroy any data to track waits.
  ~Waiter();

  // Blocks the calling thread until a matching call to `Post()` or
  // `t` has passed. Returns `true` if woken (`Post()` called),
  // `false` on timeout.
  bool Wait(KernelTimeout t);

  // Restart the caller of `Wait()` as with a normal semaphore.
  void Post();

  // If anyone is waiting, wake them up temporarily and cause them to
  // call `MaybeBecomeIdle()`. They will then return to waiting for a
  // `Post()` or timeout.
  void Poke();

  // Returns the Waiter associated with the identity.
  static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
    static_assert(
        sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
        "Insufficient space for Waiter");
    return reinterpret_cast<Waiter*>(identity->waiter_state.data);
  }

  // How many periods to remain idle before releasing resources
#ifndef ABSL_HAVE_THREAD_SANITIZER
  static constexpr int kIdlePeriods = 60;
#else
  // Memory consumption under ThreadSanitizer is a serious concern,
  // so we release resources sooner. The value of 1 leads to 1 to 2 second
  // delay before marking a thread as idle.
  static const int kIdlePeriods = 1;
#endif

 private:
#if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
  // Futexes are defined by specification to be 32-bits.
  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
  std::atomic<int32_t> futex_;
  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
  // REQUIRES: mu_ must be held.
  void InternalCondVarPoke();

  pthread_mutex_t mu_;
  pthread_cond_t cv_;
  int waiter_count_;
  int wakeup_count_;  // Unclaimed wakeups.

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
  sem_t sem_;
  // This seems superfluous, but for Poke() we need to cause spurious
  // wakeups on the semaphore. Hence we can't actually use the
  // semaphore's count.
  std::atomic<int> wakeups_;

#elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
  // WinHelper - Used to define utilities for accessing the lock and
  // condition variable storage once the types are complete.
  class WinHelper;

  // REQUIRES: WinHelper::GetLock(this) must be held.
  void InternalCondVarPoke();

  // We can't include Windows.h in our headers, so we use aligned charachter
  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
  int waiter_count_;
  int wakeup_count_;

#else
  #error Unknown ABSL_WAITER_MODE
#endif
};

}  // namespace synchronization_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_