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| 1 | +//! A `Barrier` that provides `wait_timeout`. |
| 2 | +//! |
| 3 | +//! This implementation mirrors that of the Rust standard library. |
| 4 | +
|
| 5 | +use crate::loom::sync::{Condvar, Mutex}; |
| 6 | +use std::fmt; |
| 7 | +use std::time::{Duration, Instant}; |
| 8 | + |
| 9 | +/// A barrier enables multiple threads to synchronize the beginning |
| 10 | +/// of some computation. |
| 11 | +/// |
| 12 | +/// # Examples |
| 13 | +/// |
| 14 | +/// ``` |
| 15 | +/// use std::sync::{Arc, Barrier}; |
| 16 | +/// use std::thread; |
| 17 | +/// |
| 18 | +/// let mut handles = Vec::with_capacity(10); |
| 19 | +/// let barrier = Arc::new(Barrier::new(10)); |
| 20 | +/// for _ in 0..10 { |
| 21 | +/// let c = Arc::clone(&barrier); |
| 22 | +/// // The same messages will be printed together. |
| 23 | +/// // You will NOT see any interleaving. |
| 24 | +/// handles.push(thread::spawn(move|| { |
| 25 | +/// println!("before wait"); |
| 26 | +/// c.wait(); |
| 27 | +/// println!("after wait"); |
| 28 | +/// })); |
| 29 | +/// } |
| 30 | +/// // Wait for other threads to finish. |
| 31 | +/// for handle in handles { |
| 32 | +/// handle.join().unwrap(); |
| 33 | +/// } |
| 34 | +/// ``` |
| 35 | +pub(crate) struct Barrier { |
| 36 | + lock: Mutex<BarrierState>, |
| 37 | + cvar: Condvar, |
| 38 | + num_threads: usize, |
| 39 | +} |
| 40 | + |
| 41 | +// The inner state of a double barrier |
| 42 | +struct BarrierState { |
| 43 | + count: usize, |
| 44 | + generation_id: usize, |
| 45 | +} |
| 46 | + |
| 47 | +/// A `BarrierWaitResult` is returned by [`Barrier::wait()`] when all threads |
| 48 | +/// in the [`Barrier`] have rendezvoused. |
| 49 | +/// |
| 50 | +/// # Examples |
| 51 | +/// |
| 52 | +/// ``` |
| 53 | +/// use std::sync::Barrier; |
| 54 | +/// |
| 55 | +/// let barrier = Barrier::new(1); |
| 56 | +/// let barrier_wait_result = barrier.wait(); |
| 57 | +/// ``` |
| 58 | +pub(crate) struct BarrierWaitResult(bool); |
| 59 | + |
| 60 | +impl fmt::Debug for Barrier { |
| 61 | + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 62 | + f.debug_struct("Barrier").finish_non_exhaustive() |
| 63 | + } |
| 64 | +} |
| 65 | + |
| 66 | +impl Barrier { |
| 67 | + /// Creates a new barrier that can block a given number of threads. |
| 68 | + /// |
| 69 | + /// A barrier will block `n`-1 threads which call [`wait()`] and then wake |
| 70 | + /// up all threads at once when the `n`th thread calls [`wait()`]. |
| 71 | + /// |
| 72 | + /// [`wait()`]: Barrier::wait |
| 73 | + /// |
| 74 | + /// # Examples |
| 75 | + /// |
| 76 | + /// ``` |
| 77 | + /// use std::sync::Barrier; |
| 78 | + /// |
| 79 | + /// let barrier = Barrier::new(10); |
| 80 | + /// ``` |
| 81 | + #[must_use] |
| 82 | + pub(crate) fn new(n: usize) -> Barrier { |
| 83 | + Barrier { |
| 84 | + lock: Mutex::new(BarrierState { |
| 85 | + count: 0, |
| 86 | + generation_id: 0, |
| 87 | + }), |
| 88 | + cvar: Condvar::new(), |
| 89 | + num_threads: n, |
| 90 | + } |
| 91 | + } |
| 92 | + |
| 93 | + /// Blocks the current thread until all threads have rendezvoused here. |
| 94 | + /// |
| 95 | + /// Barriers are re-usable after all threads have rendezvoused once, and can |
| 96 | + /// be used continuously. |
| 97 | + /// |
| 98 | + /// A single (arbitrary) thread will receive a [`BarrierWaitResult`] that |
| 99 | + /// returns `true` from [`BarrierWaitResult::is_leader()`] when returning |
| 100 | + /// from this function, and all other threads will receive a result that |
| 101 | + /// will return `false` from [`BarrierWaitResult::is_leader()`]. |
| 102 | + /// |
| 103 | + /// # Examples |
| 104 | + /// |
| 105 | + /// ``` |
| 106 | + /// use std::sync::{Arc, Barrier}; |
| 107 | + /// use std::thread; |
| 108 | + /// |
| 109 | + /// let mut handles = Vec::with_capacity(10); |
| 110 | + /// let barrier = Arc::new(Barrier::new(10)); |
| 111 | + /// for _ in 0..10 { |
| 112 | + /// let c = Arc::clone(&barrier); |
| 113 | + /// // The same messages will be printed together. |
| 114 | + /// // You will NOT see any interleaving. |
| 115 | + /// handles.push(thread::spawn(move|| { |
| 116 | + /// println!("before wait"); |
| 117 | + /// c.wait(); |
| 118 | + /// println!("after wait"); |
| 119 | + /// })); |
| 120 | + /// } |
| 121 | + /// // Wait for other threads to finish. |
| 122 | + /// for handle in handles { |
| 123 | + /// handle.join().unwrap(); |
| 124 | + /// } |
| 125 | + /// ``` |
| 126 | + pub(crate) fn wait(&self) -> BarrierWaitResult { |
| 127 | + let mut lock = self.lock.lock(); |
| 128 | + let local_gen = lock.generation_id; |
| 129 | + lock.count += 1; |
| 130 | + if lock.count < self.num_threads { |
| 131 | + // We need a while loop to guard against spurious wakeups. |
| 132 | + // https://en.wikipedia.org/wiki/Spurious_wakeup |
| 133 | + while local_gen == lock.generation_id { |
| 134 | + lock = self.cvar.wait(lock).unwrap(); |
| 135 | + } |
| 136 | + BarrierWaitResult(false) |
| 137 | + } else { |
| 138 | + lock.count = 0; |
| 139 | + lock.generation_id = lock.generation_id.wrapping_add(1); |
| 140 | + self.cvar.notify_all(); |
| 141 | + BarrierWaitResult(true) |
| 142 | + } |
| 143 | + } |
| 144 | + |
| 145 | + /// Blocks the current thread until all threads have rendezvoused here for |
| 146 | + /// at most `timeout` duration. |
| 147 | + pub(crate) fn wait_timeout(&self, timeout: Duration) -> Option<BarrierWaitResult> { |
| 148 | + // This implementation mirrors `wait`, but with each blocking operation |
| 149 | + // replaced by a timeout-amenable alternative. |
| 150 | + |
| 151 | + let deadline = Instant::now() + timeout; |
| 152 | + |
| 153 | + // Acquire `self.lock` with at most `timeout` duration. |
| 154 | + let mut lock = loop { |
| 155 | + if let Some(guard) = self.lock.try_lock() { |
| 156 | + break guard; |
| 157 | + } else if Instant::now() > deadline { |
| 158 | + return None; |
| 159 | + } else { |
| 160 | + std::thread::yield_now(); |
| 161 | + } |
| 162 | + }; |
| 163 | + |
| 164 | + // Shrink the `timeout` to account for the time taken to acquire `lock`. |
| 165 | + let timeout = deadline.saturating_duration_since(Instant::now()); |
| 166 | + |
| 167 | + let local_gen = lock.generation_id; |
| 168 | + lock.count += 1; |
| 169 | + if lock.count < self.num_threads { |
| 170 | + // We need a while loop to guard against spurious wakeups. |
| 171 | + // https://en.wikipedia.org/wiki/Spurious_wakeup |
| 172 | + while local_gen == lock.generation_id { |
| 173 | + let (guard, timeout_result) = self.cvar.wait_timeout(lock, timeout).unwrap(); |
| 174 | + lock = guard; |
| 175 | + if timeout_result.timed_out() { |
| 176 | + return None; |
| 177 | + } |
| 178 | + } |
| 179 | + Some(BarrierWaitResult(false)) |
| 180 | + } else { |
| 181 | + lock.count = 0; |
| 182 | + lock.generation_id = lock.generation_id.wrapping_add(1); |
| 183 | + self.cvar.notify_all(); |
| 184 | + Some(BarrierWaitResult(true)) |
| 185 | + } |
| 186 | + } |
| 187 | +} |
| 188 | + |
| 189 | +impl fmt::Debug for BarrierWaitResult { |
| 190 | + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 191 | + f.debug_struct("BarrierWaitResult") |
| 192 | + .field("is_leader", &self.is_leader()) |
| 193 | + .finish() |
| 194 | + } |
| 195 | +} |
| 196 | + |
| 197 | +impl BarrierWaitResult { |
| 198 | + /// Returns `true` if this thread is the "leader thread" for the call to |
| 199 | + /// [`Barrier::wait()`]. |
| 200 | + /// |
| 201 | + /// Only one thread will have `true` returned from their result, all other |
| 202 | + /// threads will have `false` returned. |
| 203 | + /// |
| 204 | + /// # Examples |
| 205 | + /// |
| 206 | + /// ``` |
| 207 | + /// use std::sync::Barrier; |
| 208 | + /// |
| 209 | + /// let barrier = Barrier::new(1); |
| 210 | + /// let barrier_wait_result = barrier.wait(); |
| 211 | + /// println!("{:?}", barrier_wait_result.is_leader()); |
| 212 | + /// ``` |
| 213 | + #[must_use] |
| 214 | + pub(crate) fn is_leader(&self) -> bool { |
| 215 | + self.0 |
| 216 | + } |
| 217 | +} |
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