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
#[cfg(test)]
mod tests;

use crate::fmt;
use crate::sync::{Condvar, Mutex};

/// A barrier enables multiple threads to synchronize the beginning
/// of some computation.
///
/// # Examples
///
/// ```
/// use std::sync::{Arc, Barrier};
/// use std::thread;
///
/// let mut handles = Vec::with_capacity(10);
/// let barrier = Arc::new(Barrier::new(10));
/// for _ in 0..10 {
///     let c = Arc::clone(&barrier);
///     // The same messages will be printed together.
///     // You will NOT see any interleaving.
///     handles.push(thread::spawn(move|| {
///         println!("before wait");
///         c.wait();
///         println!("after wait");
///     }));
/// }
/// // Wait for other threads to finish.
/// for handle in handles {
///     handle.join().unwrap();
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Barrier {
    lock: Mutex<BarrierState>,
    cvar: Condvar,
    num_threads: usize,
}

// The inner state of a double barrier
struct BarrierState {
    count: usize,
    generation_id: usize,
}

/// A `BarrierWaitResult` is returned by [`Barrier::wait()`] when all threads
/// in the [`Barrier`] have rendezvoused.
///
/// # Examples
///
/// ```
/// use std::sync::Barrier;
///
/// let barrier = Barrier::new(1);
/// let barrier_wait_result = barrier.wait();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct BarrierWaitResult(bool);

#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for Barrier {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Barrier").finish_non_exhaustive()
    }
}

impl Barrier {
    /// Creates a new barrier that can block a given number of threads.
    ///
    /// A barrier will block `n`-1 threads which call [`wait()`] and then wake
    /// up all threads at once when the `n`th thread calls [`wait()`].
    ///
    /// [`wait()`]: Barrier::wait
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Barrier;
    ///
    /// let barrier = Barrier::new(10);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(n: usize) -> Barrier {
        Barrier {
            lock: Mutex::new(BarrierState { count: 0, generation_id: 0 }),
            cvar: Condvar::new(),
            num_threads: n,
        }
    }

    /// Blocks the current thread until all threads have rendezvoused here.
    ///
    /// Barriers are re-usable after all threads have rendezvoused once, and can
    /// be used continuously.
    ///
    /// A single (arbitrary) thread will receive a [`BarrierWaitResult`] that
    /// returns `true` from [`BarrierWaitResult::is_leader()`] when returning
    /// from this function, and all other threads will receive a result that
    /// will return `false` from [`BarrierWaitResult::is_leader()`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Barrier};
    /// use std::thread;
    ///
    /// let mut handles = Vec::with_capacity(10);
    /// let barrier = Arc::new(Barrier::new(10));
    /// for _ in 0..10 {
    ///     let c = Arc::clone(&barrier);
    ///     // The same messages will be printed together.
    ///     // You will NOT see any interleaving.
    ///     handles.push(thread::spawn(move|| {
    ///         println!("before wait");
    ///         c.wait();
    ///         println!("after wait");
    ///     }));
    /// }
    /// // Wait for other threads to finish.
    /// for handle in handles {
    ///     handle.join().unwrap();
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn wait(&self) -> BarrierWaitResult {
        let mut lock = self.lock.lock().unwrap();
        let local_gen = lock.generation_id;
        lock.count += 1;
        if lock.count < self.num_threads {
            // We need a while loop to guard against spurious wakeups.
            // https://en.wikipedia.org/wiki/Spurious_wakeup
            while local_gen == lock.generation_id && lock.count < self.num_threads {
                lock = self.cvar.wait(lock).unwrap();
            }
            BarrierWaitResult(false)
        } else {
            lock.count = 0;
            lock.generation_id = lock.generation_id.wrapping_add(1);
            self.cvar.notify_all();
            BarrierWaitResult(true)
        }
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for BarrierWaitResult {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("BarrierWaitResult").field("is_leader", &self.is_leader()).finish()
    }
}

impl BarrierWaitResult {
    /// Returns `true` if this thread is the "leader thread" for the call to
    /// [`Barrier::wait()`].
    ///
    /// Only one thread will have `true` returned from their result, all other
    /// threads will have `false` returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Barrier;
    ///
    /// let barrier = Barrier::new(1);
    /// let barrier_wait_result = barrier.wait();
    /// println!("{:?}", barrier_wait_result.is_leader());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_leader(&self) -> bool {
        self.0
    }
}