// 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_BASE_INTERNAL_SPINLOCK_WAIT_H_ #define ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_ // Operations to make atomic transitions on a word, and to allow // waiting for those transitions to become possible. #include <stdint.h> #include <atomic> #include "absl/base/internal/scheduling_mode.h" namespace absl { ABSL_NAMESPACE_BEGIN namespace base_internal { // SpinLockWait() waits until it can perform one of several transitions from // "from" to "to". It returns when it performs a transition where done==true. struct SpinLockWaitTransition { uint32_t from; uint32_t to; bool done; }; // Wait until *w can transition from trans[i].from to trans[i].to for some i // satisfying 0<=i<n && trans[i].done, atomically make the transition, // then return the old value of *w. Make any other atomic transitions // where !trans[i].done, but continue waiting. // // Wakeups for threads blocked on SpinLockWait do not respect priorities. uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n, const SpinLockWaitTransition trans[], SchedulingMode scheduling_mode); // If possible, wake some thread that has called SpinLockDelay(w, ...). If `all` // is true, wake all such threads. On some systems, this may be a no-op; on // those systems, threads calling SpinLockDelay() will always wake eventually // even if SpinLockWake() is never called. void SpinLockWake(std::atomic<uint32_t> *w, bool all); // Wait for an appropriate spin delay on iteration "loop" of a // spin loop on location *w, whose previously observed value was "value". // SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick, // or may wait for a call to SpinLockWake(w). void SpinLockDelay(std::atomic<uint32_t> *w, uint32_t value, int loop, base_internal::SchedulingMode scheduling_mode); // Helper used by AbslInternalSpinLockDelay. // Returns a suggested delay in nanoseconds for iteration number "loop". int SpinLockSuggestedDelayNS(int loop); } // namespace base_internal ABSL_NAMESPACE_END } // namespace absl // In some build configurations we pass --detect-odr-violations to the // gold linker. This causes it to flag weak symbol overrides as ODR // violations. Because ODR only applies to C++ and not C, // --detect-odr-violations ignores symbols not mangled with C++ names. // By changing our extension points to be extern "C", we dodge this // check. extern "C" { void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(std::atomic<uint32_t> *w, bool all); void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)( std::atomic<uint32_t> *w, uint32_t value, int loop, absl::base_internal::SchedulingMode scheduling_mode); } inline void absl::base_internal::SpinLockWake(std::atomic<uint32_t> *w, bool all) { ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)(w, all); } inline void absl::base_internal::SpinLockDelay( std::atomic<uint32_t> *w, uint32_t value, int loop, absl::base_internal::SchedulingMode scheduling_mode) { ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay) (w, value, loop, scheduling_mode); } #endif // ABSL_BASE_INTERNAL_SPINLOCK_WAIT_H_