| 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"); |
| } |
| } |