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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 
     34 class Lock {
     35  private:
     36   enum LockState {
     37     Unlocked = 0,
     38     LockedWithoutWaiter,
     39     LockedWithWaiter,
     40   };
     41   _Atomic(LockState) state;
     42   bool process_shared;
     43 
     44  public:
     45   Lock(bool process_shared = false) {
     46     init(process_shared);
     47   }
     48 
     49   void init(bool process_shared) {
     50     atomic_init(&state, Unlocked);
     51     this->process_shared = process_shared;
     52   }
     53 
     54   void lock() {
     55     LockState old_state = Unlocked;
     56     if (__predict_true(atomic_compare_exchange_strong_explicit(&state, &old_state,
     57                          LockedWithoutWaiter, memory_order_acquire, memory_order_relaxed))) {
     58       return;
     59     }
     60     while (atomic_exchange_explicit(&state, LockedWithWaiter, memory_order_acquire) != Unlocked) {
     61       // TODO: As the critical section is brief, it is a better choice to spin a few times befor sleeping.
     62       __futex_wait_ex(&state, process_shared, LockedWithWaiter, NULL);
     63     }
     64     return;
     65   }
     66 
     67   void unlock() {
     68     if (atomic_exchange_explicit(&state, Unlocked, memory_order_release) == LockedWithWaiter) {
     69       __futex_wake_ex(&state, process_shared, 1);
     70     }
     71   }
     72 };
     73 
     74 #endif  // _BIONIC_LOCK_H
     75