Abseil Common Libraries (C++) (grcp 依赖)
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73 lines
2.4 KiB
73 lines
2.4 KiB
6 years ago
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// Copyright 2018 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// This file is a Linux-specific part of spinlock_wait.cc
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#include <linux/futex.h>
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#include <sys/syscall.h>
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#include <unistd.h>
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#include <atomic>
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#include <cerrno>
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#include <climits>
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#include <cstdint>
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#include <ctime>
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#include "absl/base/attributes.h"
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// The SpinLock lockword is `std::atomic<uint32_t>`. Here we assert that
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// `std::atomic<uint32_t>` is bitwise equivalent of the `int` expected
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// by SYS_futex. We also assume that reads/writes done to the lockword
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// by SYS_futex have rational semantics with regard to the
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// std::atomic<> API. C++ provides no guarantees of these assumptions,
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// but they are believed to hold in practice.
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static_assert(sizeof(std::atomic<uint32_t>) == sizeof(int),
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"SpinLock lockword has the wrong size for a futex");
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// Some Android headers are missing these definitions even though they
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// support these futex operations.
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#ifdef __BIONIC__
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#ifndef SYS_futex
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#define SYS_futex __NR_futex
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#endif
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#ifndef FUTEX_PRIVATE_FLAG
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#define FUTEX_PRIVATE_FLAG 128
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#endif
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#endif
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extern "C" {
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ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
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std::atomic<uint32_t> *w, uint32_t value, int loop,
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absl::base_internal::SchedulingMode) {
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if (loop != 0) {
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int save_errno = errno;
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struct timespec tm;
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tm.tv_sec = 0;
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// Increase the delay; we expect (but do not rely on) explicit wakeups.
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// We don't rely on explicit wakeups because we intentionally allow for
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// a race on the kSpinLockSleeper bit.
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tm.tv_nsec = 16 * absl::base_internal::SpinLockSuggestedDelayNS(loop);
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syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, &tm);
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errno = save_errno;
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}
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}
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ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(std::atomic<uint32_t> *w,
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bool all) {
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syscall(SYS_futex, w, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, all ? INT_MAX : 1, 0);
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}
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} // extern "C"
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