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
26 namespace absl {
27 ABSL_NAMESPACE_BEGIN
28 namespace 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.
32 struct 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.
44 uint32_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.
52 void 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).
58 void 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".
63 int SpinLockSuggestedDelayNS(int loop);
64
65 } // namespace base_internal
66 ABSL_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.
75 extern "C" {
76 void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(std::atomic<uint32_t> *w,
77 bool all);
78 void 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
SpinLockWake(std::atomic<uint32_t> * w,bool all)83 inline void absl::base_internal::SpinLockWake(std::atomic<uint32_t> *w,
84 bool all) {
85 ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(w, all);
86 }
87
SpinLockDelay(std::atomic<uint32_t> * w,uint32_t value,int loop,absl::base_internal::SchedulingMode scheduling_mode)88 inline 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