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
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
use core::sync::atomic::{AtomicBool, Ordering, spin_loop_hint as cpu_relax};
use core::cell::UnsafeCell;
use core::marker::Sync;
use core::ops::{Drop, Deref, DerefMut};
use core::fmt;
use core::option::Option::{self, None, Some};
use core::default::Default;

/// This type provides MUTual EXclusion based on spinning.
///
/// # Description
///
/// The behaviour of these lock is similar to their namesakes in `std::sync`. they
/// differ on the following:
///
/// - The lock will not be poisoned in case of failure;
///
/// # Simple examples
///
/// ```
/// use spin;
/// let spin_mutex = spin::Mutex::new(0);
///
/// // Modify the data
/// {
///     let mut data = spin_mutex.lock();
///     *data = 2;
/// }
///
/// // Read the data
/// let answer =
/// {
///     let data = spin_mutex.lock();
///     *data
/// };
///
/// assert_eq!(answer, 2);
/// ```
///
/// # Thread-safety example
///
/// ```
/// use spin;
/// use std::sync::{Arc, Barrier};
///
/// let numthreads = 1000;
/// let spin_mutex = Arc::new(spin::Mutex::new(0));
///
/// // We use a barrier to ensure the readout happens after all writing
/// let barrier = Arc::new(Barrier::new(numthreads + 1));
///
/// for _ in (0..numthreads)
/// {
///     let my_barrier = barrier.clone();
///     let my_lock = spin_mutex.clone();
///     std::thread::spawn(move||
///     {
///         let mut guard = my_lock.lock();
///         *guard += 1;
///
///         // Release the lock to prevent a deadlock
///         drop(guard);
///         my_barrier.wait();
///     });
/// }
///
/// barrier.wait();
///
/// let answer = { *spin_mutex.lock() };
/// assert_eq!(answer, numthreads);
/// ```
pub struct Mutex<T: ?Sized>
{
    lock: AtomicBool,
    data: UnsafeCell<T>,
}

/// A guard to which the protected data can be accessed
///
/// When the guard falls out of scope it will release the lock.
#[derive(Debug)]
pub struct MutexGuard<'a, T: ?Sized + 'a>
{
    lock: &'a AtomicBool,
    data: &'a mut T,
}

// Same unsafe impls as `std::sync::Mutex`
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}

impl<T> Mutex<T>
{
    /// Creates a new spinlock wrapping the supplied data.
    ///
    /// May be used statically:
    ///
    /// ```
    /// use spin;
    ///
    /// static MUTEX: spin::Mutex<()> = spin::Mutex::new(());
    ///
    /// fn demo() {
    ///     let lock = MUTEX.lock();
    ///     // do something with lock
    ///     drop(lock);
    /// }
    /// ```
    pub const fn new(user_data: T) -> Mutex<T>
    {
        Mutex
        {
            lock: AtomicBool::new(false),
            data: UnsafeCell::new(user_data),
        }
    }

    /// Consumes this mutex, returning the underlying data.
    pub fn into_inner(self) -> T {
        // We know statically that there are no outstanding references to
        // `self` so there's no need to lock.
        let Mutex { data, .. } = self;
        data.into_inner()
    }
}

impl<T: ?Sized> Mutex<T>
{
    fn obtain_lock(&self)
    {
        while self.lock.compare_and_swap(false, true, Ordering::Acquire) != false
        {
            // Wait until the lock looks unlocked before retrying
            while self.lock.load(Ordering::Relaxed)
            {
                cpu_relax();
            }
        }
    }

    /// Locks the spinlock and returns a guard.
    ///
    /// The returned value may be dereferenced for data access
    /// and the lock will be dropped when the guard falls out of scope.
    ///
    /// ```
    /// let mylock = spin::Mutex::new(0);
    /// {
    ///     let mut data = mylock.lock();
    ///     // The lock is now locked and the data can be accessed
    ///     *data += 1;
    ///     // The lock is implicitly dropped
    /// }
    ///
    /// ```
    pub fn lock(&self) -> MutexGuard<T>
    {
        self.obtain_lock();
        MutexGuard
        {
            lock: &self.lock,
            data: unsafe { &mut *self.data.get() },
        }
    }

    /// Force unlock the spinlock.
    ///
    /// This is *extremely* unsafe if the lock is not held by the current
    /// thread. However, this can be useful in some instances for exposing the
    /// lock to FFI that doesn't know how to deal with RAII.
    ///
    /// If the lock isn't held, this is a no-op.
    pub unsafe fn force_unlock(&self) {
        self.lock.store(false, Ordering::Release);
    }

    /// Tries to lock the mutex. If it is already locked, it will return None. Otherwise it returns
    /// a guard within Some.
    pub fn try_lock(&self) -> Option<MutexGuard<T>>
    {
        if self.lock.compare_and_swap(false, true, Ordering::Acquire) == false
        {
            Some(
                MutexGuard {
                    lock: &self.lock,
                    data: unsafe { &mut *self.data.get() },
                }
            )
        }
        else
        {
            None
        }
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T>
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result
    {
        match self.try_lock()
        {
            Some(guard) => write!(f, "Mutex {{ data: ")
				.and_then(|()| (&*guard).fmt(f))
				.and_then(|()| write!(f, "}}")),
            None => write!(f, "Mutex {{ <locked> }}"),
        }
    }
}

impl<T: ?Sized + Default> Default for Mutex<T> {
    fn default() -> Mutex<T> {
        Mutex::new(Default::default())
    }
}

impl<'a, T: ?Sized> Deref for MutexGuard<'a, T>
{
    type Target = T;
    fn deref<'b>(&'b self) -> &'b T { &*self.data }
}

impl<'a, T: ?Sized> DerefMut for MutexGuard<'a, T>
{
    fn deref_mut<'b>(&'b mut self) -> &'b mut T { &mut *self.data }
}

impl<'a, T: ?Sized> Drop for MutexGuard<'a, T>
{
    /// The dropping of the MutexGuard will release the lock it was created from.
    fn drop(&mut self)
    {
        self.lock.store(false, Ordering::Release);
    }
}

#[cfg(test)]
mod tests {
    use std::prelude::v1::*;

    use std::sync::mpsc::channel;
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::thread;

    use super::*;

    #[derive(Eq, PartialEq, Debug)]
    struct NonCopy(i32);

    #[test]
    fn smoke() {
        let m = Mutex::new(());
        drop(m.lock());
        drop(m.lock());
    }

    #[test]
    fn lots_and_lots() {
        static M: Mutex<()>  = Mutex::new(());
        static mut CNT: u32 = 0;
        const J: u32 = 1000;
        const K: u32 = 3;

        fn inc() {
            for _ in 0..J {
                unsafe {
                    let _g = M.lock();
                    CNT += 1;
                }
            }
        }

        let (tx, rx) = channel();
        for _ in 0..K {
            let tx2 = tx.clone();
            thread::spawn(move|| { inc(); tx2.send(()).unwrap(); });
            let tx2 = tx.clone();
            thread::spawn(move|| { inc(); tx2.send(()).unwrap(); });
        }

        drop(tx);
        for _ in 0..2 * K {
            rx.recv().unwrap();
        }
        assert_eq!(unsafe {CNT}, J * K * 2);
    }

    #[test]
    fn try_lock() {
        let mutex = Mutex::new(42);

        // First lock succeeds
        let a = mutex.try_lock();
        assert_eq!(a.as_ref().map(|r| **r), Some(42));

        // Additional lock failes
        let b = mutex.try_lock();
        assert!(b.is_none());

        // After dropping lock, it succeeds again
        ::core::mem::drop(a);
        let c = mutex.try_lock();
        assert_eq!(c.as_ref().map(|r| **r), Some(42));
    }

    #[test]
    fn test_into_inner() {
        let m = Mutex::new(NonCopy(10));
        assert_eq!(m.into_inner(), NonCopy(10));
    }

    #[test]
    fn test_into_inner_drop() {
        struct Foo(Arc<AtomicUsize>);
        impl Drop for Foo {
            fn drop(&mut self) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }
        let num_drops = Arc::new(AtomicUsize::new(0));
        let m = Mutex::new(Foo(num_drops.clone()));
        assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        {
            let _inner = m.into_inner();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_mutex_arc_nested() {
        // Tests nested mutexes and access
        // to underlying data.
        let arc = Arc::new(Mutex::new(1));
        let arc2 = Arc::new(Mutex::new(arc));
        let (tx, rx) = channel();
        let _t = thread::spawn(move|| {
            let lock = arc2.lock();
            let lock2 = lock.lock();
            assert_eq!(*lock2, 1);
            tx.send(()).unwrap();
        });
        rx.recv().unwrap();
    }

    #[test]
    fn test_mutex_arc_access_in_unwind() {
        let arc = Arc::new(Mutex::new(1));
        let arc2 = arc.clone();
        let _ = thread::spawn(move|| -> () {
            struct Unwinder {
                i: Arc<Mutex<i32>>,
            }
            impl Drop for Unwinder {
                fn drop(&mut self) {
                    *self.i.lock() += 1;
                }
            }
            let _u = Unwinder { i: arc2 };
            panic!();
        }).join();
        let lock = arc.lock();
        assert_eq!(*lock, 2);
    }

    #[test]
    fn test_mutex_unsized() {
        let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
        {
            let b = &mut *mutex.lock();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*mutex.lock(), comp);
    }

    #[test]
    fn test_mutex_force_lock() {
        let lock = Mutex::new(());
        ::std::mem::forget(lock.lock());
        unsafe {
            lock.force_unlock();
        } 
        assert!(lock.try_lock().is_some());
    }
}