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      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 #ifndef _BIONIC_LOCK_H
     29 #define _BIONIC_LOCK_H
     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:
     48   void init(bool process_shared) {
     49     atomic_init(&state, Unlocked);
     50     this->process_shared = process_shared;
     51   }
     52 
     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 
     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 
     72   void unlock() {
     73     if (atomic_exchange_explicit(&state, Unlocked, memory_order_release) == LockedWithWaiter) {
     74       __futex_wake_ex(&state, process_shared, 1);
     75     }
     76   }
     77 };
     78 
     79 class LockGuard {
     80  public:
     81   LockGuard(Lock& lock) : lock_(lock) {
     82     lock_.lock();
     83   }
     84   ~LockGuard() {
     85     lock_.unlock();
     86   }
     87 
     88   DISALLOW_COPY_AND_ASSIGN(LockGuard);
     89 
     90  private:
     91   Lock& lock_;
     92 };
     93 
     94 #endif  // _BIONIC_LOCK_H
     95