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
32#include "absl/base/internal/thread_identity.h"
33#include "absl/synchronization/internal/kernel_timeout.h"
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
53namespace absl {
54namespace synchronization_internal {
55
56// Waiter is an OS-specific semaphore.
57class 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.
82 static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
83 static_assert(
84 sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
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_
150