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use core::mem;
use core::sync::atomic::{self, AtomicUsize, Ordering};
use crate::Backoff;
/// A simple stamped lock.
pub(crate) 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(crate) const fn new() -> Self {
Self {
state: AtomicUsize::new(0),
}
}
/// If not locked, returns the current stamp.
///
/// This method should be called before optimistic reads.
#[inline]
pub(crate) 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(crate) fn validate_read(&self, stamp: usize) -> bool {
atomic::fence(Ordering::Acquire);
self.state.load(Ordering::Relaxed) == stamp
}
/// Grabs the lock for writing.
#[inline]
pub(crate) 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(crate) 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(crate) fn abort(self) {
self.lock.state.store(self.state, Ordering::Release);
// We specifically don't want to call drop(), since that's
// what increments the stamp.
mem::forget(self);
}
}
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);
}
}
#[cfg(test)]
mod tests {
use super::SeqLock;
#[test]
fn test_abort() {
static LK: SeqLock = SeqLock::new();
let before = LK.optimistic_read().unwrap();
{
let guard = LK.write();
guard.abort();
}
let after = LK.optimistic_read().unwrap();
assert_eq!(before, after, "aborted write does not update the stamp");
}
}