1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License..

use super::wait::{futex_wait, futex_wake};
use core::sync::atomic::{
    AtomicU32,
    Ordering::{Acquire, Relaxed, Release},
};

pub type MovableMutex = Mutex;

pub struct Mutex {
    /// 0: unlocked
    /// 1: locked, no other threads waiting
    /// 2: locked, and other threads waiting (contended)
    futex: AtomicU32,
}

impl Mutex {
    #[inline]
    pub const fn new() -> Self {
        Self {
            futex: AtomicU32::new(0),
        }
    }

    #[inline]
    pub unsafe fn try_lock(&self) -> bool {
        self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()
    }

    #[inline]
    pub unsafe fn lock(&self) {
        if self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_err() {
            self.lock_contended();
        }
    }

    #[cold]
    fn lock_contended(&self) {
        // Spin first to speed things up if the lock is released quickly.
        let mut state = self.spin();

        // If it's unlocked now, attempt to take the lock
        // without marking it as contended.
        if state == 0 {
            match self.futex.compare_exchange(0, 1, Acquire, Relaxed) {
                Ok(_) => return, // Locked!
                Err(s) => state = s,
            }
        }

        loop {
            // Put the lock in contended state.
            // We avoid an unnecessary write if it as already set to 2,
            // to be friendlier for the caches.
            if state != 2 && self.futex.swap(2, Acquire) == 0 {
                // We changed it from 0 to 2, so we just successfully locked it.
                return;
            }

            // Wait for the futex to change state, assuming it is still 2.
            futex_wait(&self.futex, 2, None);

            // Spin again after waking up.
            state = self.spin();
        }
    }

    fn spin(&self) -> u32 {
        // In rust's implmenetation, this is `100`. Considering more overhead in SGX environment,
        // here we make it bigger.
        let mut spin = 1000;
        loop {
            // We only use `load` (and not `swap` or `compare_exchange`)
            // while spinning, to be easier on the caches.
            let state = self.futex.load(Relaxed);

            // We stop spinning when the mutex is unlocked (0),
            // but also when it's contended (2).
            if state != 1 || spin == 0 {
                return state;
            }

            core::hint::spin_loop();
            spin -= 1;
        }
    }

    #[inline]
    pub unsafe fn unlock(&self) {
        if self.futex.swap(0, Release) == 2 {
            // We only wake up one thread. When that thread locks the mutex, it
            // will mark the mutex as contended (2) (see lock_contended above),
            // which makes sure that any other waiting threads will also be
            // woken up eventually.
            self.wake();
        }
    }

    #[cold]
    fn wake(&self) {
        futex_wake(&self.futex);
    }
}