summaryrefslogtreecommitdiffstats
path: root/library/std/src/sync/remutex.rs
blob: 519ec2c32bd5d5b59ec50b5699851e141cd9f6f3 (plain)
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
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;

use crate::cell::UnsafeCell;
use crate::ops::Deref;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
use crate::sys::locks as sys;

/// A re-entrant mutual exclusion
///
/// This mutex will block *other* threads waiting for the lock to become
/// available. The thread which has already locked the mutex can lock it
/// multiple times without blocking, preventing a common source of deadlocks.
///
/// This is used by stdout().lock() and friends.
///
/// ## Implementation details
///
/// The 'owner' field tracks which thread has locked the mutex.
///
/// We use current_thread_unique_ptr() as the thread identifier,
/// which is just the address of a thread local variable.
///
/// If `owner` is set to the identifier of the current thread,
/// we assume the mutex is already locked and instead of locking it again,
/// we increment `lock_count`.
///
/// When unlocking, we decrement `lock_count`, and only unlock the mutex when
/// it reaches zero.
///
/// `lock_count` is protected by the mutex and only accessed by the thread that has
/// locked the mutex, so needs no synchronization.
///
/// `owner` can be checked by other threads that want to see if they already
/// hold the lock, so needs to be atomic. If it compares equal, we're on the
/// same thread that holds the mutex and memory access can use relaxed ordering
/// since we're not dealing with multiple threads. If it's not equal,
/// synchronization is left to the mutex, making relaxed memory ordering for
/// the `owner` field fine in all cases.
pub struct ReentrantMutex<T> {
    mutex: sys::Mutex,
    owner: AtomicUsize,
    lock_count: UnsafeCell<u32>,
    data: T,
}

unsafe impl<T: Send> Send for ReentrantMutex<T> {}
unsafe impl<T: Send> Sync for ReentrantMutex<T> {}

impl<T> UnwindSafe for ReentrantMutex<T> {}
impl<T> RefUnwindSafe for ReentrantMutex<T> {}

/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
/// dropped (falls out of scope), the lock will be unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// Deref implementation.
///
/// # Mutability
///
/// Unlike `MutexGuard`, `ReentrantMutexGuard` does not implement `DerefMut`,
/// because implementation of the trait would violate Rust’s reference aliasing
/// rules. Use interior mutability (usually `RefCell`) in order to mutate the
/// guarded data.
#[must_use = "if unused the ReentrantMutex will immediately unlock"]
pub struct ReentrantMutexGuard<'a, T: 'a> {
    lock: &'a ReentrantMutex<T>,
}

impl<T> !Send for ReentrantMutexGuard<'_, T> {}

impl<T> ReentrantMutex<T> {
    /// Creates a new reentrant mutex in an unlocked state.
    pub const fn new(t: T) -> ReentrantMutex<T> {
        ReentrantMutex {
            mutex: sys::Mutex::new(),
            owner: AtomicUsize::new(0),
            lock_count: UnsafeCell::new(0),
            data: t,
        }
    }

    /// Acquires a mutex, blocking the current thread until it is able to do so.
    ///
    /// This function will block the caller until it is available to acquire the mutex.
    /// Upon returning, the thread is the only thread with the mutex held. When the thread
    /// calling this method already holds the lock, the call shall succeed without
    /// blocking.
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return failure if the mutex would otherwise be
    /// acquired.
    pub fn lock(&self) -> ReentrantMutexGuard<'_, T> {
        let this_thread = current_thread_unique_ptr();
        // Safety: We only touch lock_count when we own the lock.
        unsafe {
            if self.owner.load(Relaxed) == this_thread {
                self.increment_lock_count();
            } else {
                self.mutex.lock();
                self.owner.store(this_thread, Relaxed);
                debug_assert_eq!(*self.lock_count.get(), 0);
                *self.lock_count.get() = 1;
            }
        }
        ReentrantMutexGuard { lock: self }
    }

    /// Attempts to acquire this lock.
    ///
    /// If the lock could not be acquired at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned.
    ///
    /// This function does not block.
    ///
    /// # Errors
    ///
    /// If another user of this mutex panicked while holding the mutex, then
    /// this call will return failure if the mutex would otherwise be
    /// acquired.
    pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, T>> {
        let this_thread = current_thread_unique_ptr();
        // Safety: We only touch lock_count when we own the lock.
        unsafe {
            if self.owner.load(Relaxed) == this_thread {
                self.increment_lock_count();
                Some(ReentrantMutexGuard { lock: self })
            } else if self.mutex.try_lock() {
                self.owner.store(this_thread, Relaxed);
                debug_assert_eq!(*self.lock_count.get(), 0);
                *self.lock_count.get() = 1;
                Some(ReentrantMutexGuard { lock: self })
            } else {
                None
            }
        }
    }

    unsafe fn increment_lock_count(&self) {
        *self.lock_count.get() = (*self.lock_count.get())
            .checked_add(1)
            .expect("lock count overflow in reentrant mutex");
    }
}

impl<T> Deref for ReentrantMutexGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.lock.data
    }
}

impl<T> Drop for ReentrantMutexGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        // Safety: We own the lock.
        unsafe {
            *self.lock.lock_count.get() -= 1;
            if *self.lock.lock_count.get() == 0 {
                self.lock.owner.store(0, Relaxed);
                self.lock.mutex.unlock();
            }
        }
    }
}

/// Get an address that is unique per running thread.
///
/// This can be used as a non-null usize-sized ID.
pub fn current_thread_unique_ptr() -> usize {
    // Use a non-drop type to make sure it's still available during thread destruction.
    thread_local! { static X: u8 = const { 0 } }
    X.with(|x| <*const _>::addr(x))
}