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use core::sync::atomic::{self, AtomicUsize, Ordering};
use Backoff;
/// A simple stamped lock.
pub struct SeqLock {
/// The current state of the lock.
///
/// All bits except the least significant one hold the current stamp. When locked, the state
/// equals 1 and doesn't contain a valid stamp.
state: AtomicUsize,
}
impl SeqLock {
pub const INIT: Self = Self {
state: AtomicUsize::new(0),
};
/// If not locked, returns the current stamp.
///
/// This method should be called before optimistic reads.
#[inline]
pub fn optimistic_read(&self) -> Option<usize> {
let state = self.state.load(Ordering::Acquire);
if state == 1 {
None
} else {
Some(state)
}
}
/// Returns `true` if the current stamp is equal to `stamp`.
///
/// This method should be called after optimistic reads to check whether they are valid. The
/// argument `stamp` should correspond to the one returned by method `optimistic_read`.
#[inline]
pub fn validate_read(&self, stamp: usize) -> bool {
atomic::fence(Ordering::Acquire);
self.state.load(Ordering::Relaxed) == stamp
}
/// Grabs the lock for writing.
#[inline]
pub fn write(&'static self) -> SeqLockWriteGuard {
let backoff = Backoff::new();
loop {
let previous = self.state.swap(1, Ordering::Acquire);
if previous != 1 {
atomic::fence(Ordering::Release);
return SeqLockWriteGuard {
lock: self,
state: previous,
};
}
backoff.snooze();
}
}
}
/// An RAII guard that releases the lock and increments the stamp when dropped.
pub struct SeqLockWriteGuard {
/// The parent lock.
lock: &'static SeqLock,
/// The stamp before locking.
state: usize,
}
impl SeqLockWriteGuard {
/// Releases the lock without incrementing the stamp.
#[inline]
pub fn abort(self) {
self.lock.state.store(self.state, Ordering::Release);
}
}
impl Drop for SeqLockWriteGuard {
#[inline]
fn drop(&mut self) {
// Release the lock and increment the stamp.
self.lock
.state
.store(self.state.wrapping_add(2), Ordering::Release);
}
}
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