| // Copyright 2014 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| use sync::{Mutex, Condvar}; |
| |
| /// A barrier enables multiple threads to synchronize the beginning |
| /// of some computation. |
| /// |
| /// ``` |
| /// use std::sync::{Arc, Barrier}; |
| /// use std::thread; |
| /// |
| /// let barrier = Arc::new(Barrier::new(10)); |
| /// for _ in 0..10 { |
| /// let c = barrier.clone(); |
| /// // The same messages will be printed together. |
| /// // You will NOT see any interleaving. |
| /// thread::spawn(move|| { |
| /// println!("before wait"); |
| /// c.wait(); |
| /// println!("after wait"); |
| /// }); |
| /// } |
| /// ``` |
| #[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 result returned from wait. |
| /// |
| /// Currently this opaque structure only has one method, `.is_leader()`. Only |
| /// one thread will receive a result that will return `true` from this function. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub struct BarrierWaitResult(bool); |
| |
| 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`. |
| #[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 has 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 `is_leader` when returning from this function, and |
| /// all other threads will receive a result that will return `false` from |
| /// `is_leader` |
| #[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. |
| // http://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 += 1; |
| self.cvar.notify_all(); |
| BarrierWaitResult(true) |
| } |
| } |
| } |
| |
| impl BarrierWaitResult { |
| /// Returns whether this thread from `wait` is the "leader thread". |
| /// |
| /// Only one thread will have `true` returned from their result, all other |
| /// threads will have `false` returned. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub fn is_leader(&self) -> bool { self.0 } |
| } |
| |
| #[cfg(test)] |
| mod tests { |
| use prelude::v1::*; |
| |
| use sync::{Arc, Barrier}; |
| use sync::mpsc::{channel, TryRecvError}; |
| use thread; |
| |
| #[test] |
| fn test_barrier() { |
| const N: usize = 10; |
| |
| let barrier = Arc::new(Barrier::new(N)); |
| let (tx, rx) = channel(); |
| |
| for _ in 0..N - 1 { |
| let c = barrier.clone(); |
| let tx = tx.clone(); |
| thread::spawn(move|| { |
| tx.send(c.wait().is_leader()).unwrap(); |
| }); |
| } |
| |
| // At this point, all spawned threads should be blocked, |
| // so we shouldn't get anything from the port |
| assert!(match rx.try_recv() { |
| Err(TryRecvError::Empty) => true, |
| _ => false, |
| }); |
| |
| let mut leader_found = barrier.wait().is_leader(); |
| |
| // Now, the barrier is cleared and we should get data. |
| for _ in 0..N - 1 { |
| if rx.recv().unwrap() { |
| assert!(!leader_found); |
| leader_found = true; |
| } |
| } |
| assert!(leader_found); |
| } |
| } |