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.
37class 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, false, nullptr);
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#endif  // _BIONIC_LOCK_H
80