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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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				|  |  | +
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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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				|  |  | +
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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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				|  |  | +
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				|  |  | +#include "absl/base/attributes.h"
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				|  |  | +
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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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				|  |  | +
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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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				|  |  | +
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				|  |  | +extern "C" {
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				|  |  | +
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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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				|  |  | +
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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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				|  |  | +
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				|  |  | +}  // extern "C"
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