100 lines
3.6 KiB
C++
100 lines
3.6 KiB
C++
/*
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* Copyright (C) 2015 The Android Open Source Project
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in
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* the documentation and/or other materials provided with the
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* distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#pragma once
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#include <stdatomic.h>
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#include "private/bionic_futex.h"
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#include "platform/bionic/macros.h"
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// Lock is used in places like pthread_rwlock_t, which can be initialized without calling
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// an initialization function. So make sure Lock can be initialized by setting its memory to 0.
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class Lock {
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private:
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enum LockState {
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Unlocked = 0,
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LockedWithoutWaiter,
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LockedWithWaiter,
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};
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_Atomic(LockState) state;
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bool process_shared;
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public:
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void init(bool process_shared) {
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atomic_init(&state, Unlocked);
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this->process_shared = process_shared;
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}
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bool trylock() {
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LockState old_state = Unlocked;
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return __predict_true(atomic_compare_exchange_strong_explicit(&state, &old_state,
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LockedWithoutWaiter, memory_order_acquire, memory_order_relaxed));
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}
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void lock() {
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LockState old_state = Unlocked;
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if (__predict_true(atomic_compare_exchange_strong_explicit(&state, &old_state,
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LockedWithoutWaiter, memory_order_acquire, memory_order_relaxed))) {
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return;
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}
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while (atomic_exchange_explicit(&state, LockedWithWaiter, memory_order_acquire) != Unlocked) {
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// TODO: As the critical section is brief, it is a better choice to spin a few times befor sleeping.
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__futex_wait_ex(&state, process_shared, LockedWithWaiter);
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}
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return;
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}
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void unlock() {
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bool shared = process_shared; /* cache to local variable */
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if (atomic_exchange_explicit(&state, Unlocked, memory_order_release) == LockedWithWaiter) {
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// The Lock object may have been deallocated between the atomic exchange and the futex wake
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// call, so avoid accessing any fields of Lock here. In that case, the wake call may target
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// unmapped memory or trigger a spurious futex wakeup. The same situation happens with
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// pthread mutexes. References:
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// - https://lkml.org/lkml/2014/11/27/472
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// - http://austingroupbugs.net/view.php?id=811#c2267
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__futex_wake_ex(&state, shared, 1);
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}
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}
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};
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class LockGuard {
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public:
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explicit LockGuard(Lock& lock) : lock_(lock) {
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lock_.lock();
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}
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~LockGuard() {
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lock_.unlock();
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}
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BIONIC_DISALLOW_COPY_AND_ASSIGN(LockGuard);
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private:
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Lock& lock_;
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};
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