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#ifndef ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_
16#define ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_
17
18// Operations to make atomic transitions on a word, and to allow
19// waiting for those transitions to become possible.
20
21#include <stdint.h>
22#include <atomic>
23
24#include "absl/base/internal/scheduling_mode.h"
25
26namespace absl {
27ABSL_NAMESPACE_BEGIN
28namespace base_internal {
29
30// SpinLockWait() waits until it can perform one of several transitions from
31// "from" to "to". It returns when it performs a transition where done==true.
32struct SpinLockWaitTransition {
33 uint32_t from;
34 uint32_t to;
35 bool done;
36};
37
38// Wait until *w can transition from trans[i].from to trans[i].to for some i
39// satisfying 0<=i<n && trans[i].done, atomically make the transition,
40// then return the old value of *w. Make any other atomic transitions
41// where !trans[i].done, but continue waiting.
42//
43// Wakeups for threads blocked on SpinLockWait do not respect priorities.
44uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n,
45 const SpinLockWaitTransition trans[],
46 SchedulingMode scheduling_mode);
47
48// If possible, wake some thread that has called SpinLockDelay(w, ...). If `all`
49// is true, wake all such threads. On some systems, this may be a no-op; on
50// those systems, threads calling SpinLockDelay() will always wake eventually
51// even if SpinLockWake() is never called.
52void SpinLockWake(std::atomic<uint32_t> *w, bool all);
53
54// Wait for an appropriate spin delay on iteration "loop" of a
55// spin loop on location *w, whose previously observed value was "value".
56// SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick,
57// or may wait for a call to SpinLockWake(w).
58void SpinLockDelay(std::atomic<uint32_t> *w, uint32_t value, int loop,
59 base_internal::SchedulingMode scheduling_mode);
60
61// Helper used by AbslInternalSpinLockDelay.
62// Returns a suggested delay in nanoseconds for iteration number "loop".
63int SpinLockSuggestedDelayNS(int loop);
64
65} // namespace base_internal
66ABSL_NAMESPACE_END
67} // namespace absl
68
69// In some build configurations we pass --detect-odr-violations to the
70// gold linker. This causes it to flag weak symbol overrides as ODR
71// violations. Because ODR only applies to C++ and not C,
72// --detect-odr-violations ignores symbols not mangled with C++ names.
73// By changing our extension points to be extern "C", we dodge this
74// check.
75extern "C" {
76void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(std::atomic<uint32_t> *w,
77 bool all);
78void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)(
79 std::atomic<uint32_t> *w, uint32_t value, int loop,
80 absl::base_internal::SchedulingMode scheduling_mode);
81}
82
83inline void absl::base_internal::SpinLockWake(std::atomic<uint32_t> *w,
84 bool all) {
85 ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(w, all);
86}
87
88inline void absl::base_internal::SpinLockDelay(
89 std::atomic<uint32_t> *w, uint32_t value, int loop,
90 absl::base_internal::SchedulingMode scheduling_mode) {
91 ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)
92 (w, value, loop, scheduling_mode);
93}
94
95#endif // ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_
96