1 /* 2 * Copyright (C) 2015 The Android Open Source Project 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * * Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * * Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in 12 * the documentation and/or other materials provided with the 13 * distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED 23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 #pragma once 30 31 #include <stdatomic.h> 32 #include "private/bionic_futex.h" 33 #include "private/bionic_macros.h" 34 35 // Lock is used in places like pthread_rwlock_t, which can be initialized without calling 36 // an initialization function. So make sure Lock can be initialized by setting its memory to 0. 37 class Lock { 38 private: 39 enum LockState { 40 Unlocked = 0, 41 LockedWithoutWaiter, 42 LockedWithWaiter, 43 }; 44 _Atomic(LockState) state; 45 bool process_shared; 46 47 public: init(bool process_shared)48 void init(bool process_shared) { 49 atomic_init(&state, Unlocked); 50 this->process_shared = process_shared; 51 } 52 trylock()53 bool trylock() { 54 LockState old_state = Unlocked; 55 return __predict_true(atomic_compare_exchange_strong_explicit(&state, &old_state, 56 LockedWithoutWaiter, memory_order_acquire, memory_order_relaxed)); 57 } 58 lock()59 void lock() { 60 LockState old_state = Unlocked; 61 if (__predict_true(atomic_compare_exchange_strong_explicit(&state, &old_state, 62 LockedWithoutWaiter, memory_order_acquire, memory_order_relaxed))) { 63 return; 64 } 65 while (atomic_exchange_explicit(&state, LockedWithWaiter, memory_order_acquire) != Unlocked) { 66 // TODO: As the critical section is brief, it is a better choice to spin a few times befor sleeping. 67 __futex_wait_ex(&state, process_shared, LockedWithWaiter); 68 } 69 return; 70 } 71 unlock()72 void unlock() { 73 bool shared = process_shared; /* cache to local variable */ 74 if (atomic_exchange_explicit(&state, Unlocked, memory_order_release) == LockedWithWaiter) { 75 __futex_wake_ex(&state, shared, 1); 76 } 77 } 78 }; 79 80 class LockGuard { 81 public: LockGuard(Lock & lock)82 explicit LockGuard(Lock& lock) : lock_(lock) { 83 lock_.lock(); 84 } ~LockGuard()85 ~LockGuard() { 86 lock_.unlock(); 87 } 88 89 BIONIC_DISALLOW_COPY_AND_ASSIGN(LockGuard); 90 91 private: 92 Lock& lock_; 93 }; 94