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use super::Mutex;
use crate::sync::atomic::{AtomicU32, Ordering::Relaxed};
use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
use crate::time::Duration;
pub struct Condvar {
// The value of this atomic is simply incremented on every notification.
// This is used by `.wait()` to not miss any notifications after
// unlocking the mutex and before waiting for notifications.
futex: AtomicU32,
}
impl Condvar {
#[inline]
pub const fn new() -> Self {
Self { futex: AtomicU32::new(0) }
}
// All the memory orderings here are `Relaxed`,
// because synchronization is done by unlocking and locking the mutex.
pub fn notify_one(&self) {
self.futex.fetch_add(1, Relaxed);
futex_wake(&self.futex);
}
pub fn notify_all(&self) {
self.futex.fetch_add(1, Relaxed);
futex_wake_all(&self.futex);
}
pub unsafe fn wait(&self, mutex: &Mutex) {
self.wait_optional_timeout(mutex, None);
}
pub unsafe fn wait_timeout(&self, mutex: &Mutex, timeout: Duration) -> bool {
self.wait_optional_timeout(mutex, Some(timeout))
}
unsafe fn wait_optional_timeout(&self, mutex: &Mutex, timeout: Option<Duration>) -> bool {
// Examine the notification counter _before_ we unlock the mutex.
let futex_value = self.futex.load(Relaxed);
// Unlock the mutex before going to sleep.
mutex.unlock();
// Wait, but only if there hasn't been any
// notification since we unlocked the mutex.
let r = futex_wait(&self.futex, futex_value, timeout);
// Lock the mutex again.
mutex.lock();
r
}
}