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|
#[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 n = 10;
/// let mut handles = Vec::with_capacity(n);
/// let barrier = Arc::new(Barrier::new(n));
/// for _ in 0..n {
/// 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")]
#[must_use]
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 n = 10;
/// let mut handles = Vec::with_capacity(n);
/// let barrier = Arc::new(Barrier::new(n));
/// for _ in 0..n {
/// 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 {
let _guard =
self.cvar.wait_while(lock, |state| local_gen == state.generation_id).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")]
#[must_use]
pub fn is_leader(&self) -> bool {
self.0
}
}
|
