// Copyright 2018 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. // // This file is a Linux-specific part of spinlock_wait.cc #include <linux/futex.h> #include <sys/syscall.h> #include <unistd.h> #include <atomic> #include <climits> #include <cstdint> #include <ctime> #include "absl/base/attributes.h" #include "absl/base/internal/errno_saver.h" // The SpinLock lockword is `std::atomic<uint32_t>`. Here we assert that // `std::atomic<uint32_t>` is bitwise equivalent of the `int` expected // by SYS_futex. We also assume that reads/writes done to the lockword // by SYS_futex have rational semantics with regard to the // std::atomic<> API. C++ provides no guarantees of these assumptions, // but they are believed to hold in practice. static_assert(sizeof(std::atomic<uint32_t>) == sizeof(int), "SpinLock lockword has the wrong size for a futex"); // Some Android headers are missing these definitions even though they // support these futex operations. #ifdef __BIONIC__ #ifndef SYS_futex #define SYS_futex __NR_futex #endif #ifndef FUTEX_PRIVATE_FLAG #define FUTEX_PRIVATE_FLAG 128 #endif #endif #if defined(__NR_futex_time64) && !defined(SYS_futex_time64) #define SYS_futex_time64 __NR_futex_time64 #endif #if defined(SYS_futex_time64) && !defined(SYS_futex) #define SYS_futex SYS_futex_time64 #endif extern "C" { ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockDelay)( std::atomic<uint32_t> *w, uint32_t value, int loop, absl::base_internal::SchedulingMode) { absl::base_internal::ErrnoSaver errno_saver; struct timespec tm; tm.tv_sec = 0; tm.tv_nsec = absl::base_internal::SpinLockSuggestedDelayNS(loop); syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, &tm); } ABSL_ATTRIBUTE_WEAK void ABSL_INTERNAL_C_SYMBOL(AbslInternalSpinLockWake)( std::atomic<uint32_t> *w, bool all) { syscall(SYS_futex, w, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, all ? INT_MAX : 1, 0); } } // extern "C"