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Diffstat (limited to '')
-rw-r--r-- | vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs | 155 |
1 files changed, 155 insertions, 0 deletions
diff --git a/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs b/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs new file mode 100644 index 000000000..ef5d94a45 --- /dev/null +++ b/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs @@ -0,0 +1,155 @@ +use core::mem; +use core::sync::atomic::{self, AtomicUsize, Ordering}; + +use crate::Backoff; + +/// A simple stamped lock. +/// +/// The state is represented as two `AtomicUsize`: `state_hi` for high bits and `state_lo` for low +/// bits. +pub(crate) struct SeqLock { + /// The high bits of the current state of the lock. + state_hi: AtomicUsize, + + /// The low bits of the current state of the lock. + /// + /// All bits except the least significant one hold the current stamp. When locked, the state_lo + /// equals 1 and doesn't contain a valid stamp. + state_lo: AtomicUsize, +} + +impl SeqLock { + pub(crate) const fn new() -> Self { + Self { + state_hi: AtomicUsize::new(0), + state_lo: 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, usize)> { + // The acquire loads from `state_hi` and `state_lo` synchronize with the release stores in + // `SeqLockWriteGuard::drop`. + // + // As a consequence, we can make sure that (1) all writes within the era of `state_hi - 1` + // happens before now; and therefore, (2) if `state_lo` is even, all writes within the + // critical section of (`state_hi`, `state_lo`) happens before now. + let state_hi = self.state_hi.load(Ordering::Acquire); + let state_lo = self.state_lo.load(Ordering::Acquire); + if state_lo == 1 { + None + } else { + Some((state_hi, state_lo)) + } + } + + /// 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, usize)) -> bool { + // Thanks to the fence, if we're noticing any modification to the data at the critical + // section of `(a, b)`, then the critical section's write of 1 to state_lo should be + // visible. + atomic::fence(Ordering::Acquire); + + // So if `state_lo` coincides with `stamp.1`, then either (1) we're noticing no modification + // to the data after the critical section of `(stamp.0, stamp.1)`, or (2) `state_lo` wrapped + // around. + // + // If (2) is the case, the acquire ordering ensures we see the new value of `state_hi`. + let state_lo = self.state_lo.load(Ordering::Acquire); + + // If (2) is the case and `state_hi` coincides with `stamp.0`, then `state_hi` also wrapped + // around, which we give up to correctly validate the read. + let state_hi = self.state_hi.load(Ordering::Relaxed); + + // Except for the case that both `state_hi` and `state_lo` wrapped around, the following + // condition implies that we're noticing no modification to the data after the critical + // section of `(stamp.0, stamp.1)`. + (state_hi, state_lo) == stamp + } + + /// Grabs the lock for writing. + #[inline] + pub(crate) fn write(&'static self) -> SeqLockWriteGuard { + let backoff = Backoff::new(); + loop { + let previous = self.state_lo.swap(1, Ordering::Acquire); + + if previous != 1 { + // To synchronize with the acquire fence in `validate_read` via any modification to + // the data at the critical section of `(state_hi, previous)`. + atomic::fence(Ordering::Release); + + return SeqLockWriteGuard { + lock: self, + state_lo: 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_lo: usize, +} + +impl SeqLockWriteGuard { + /// Releases the lock without incrementing the stamp. + #[inline] + pub(crate) fn abort(self) { + self.lock.state_lo.store(self.state_lo, Ordering::Release); + mem::forget(self); + } +} + +impl Drop for SeqLockWriteGuard { + #[inline] + fn drop(&mut self) { + let state_lo = self.state_lo.wrapping_add(2); + + // Increase the high bits if the low bits wrap around. + // + // Release ordering for synchronizing with `optimistic_read`. + if state_lo == 0 { + let state_hi = self.lock.state_hi.load(Ordering::Relaxed); + self.lock + .state_hi + .store(state_hi.wrapping_add(1), Ordering::Release); + } + + // Release the lock and increment the stamp. + // + // Release ordering for synchronizing with `optimistic_read`. + self.lock.state_lo.store(state_lo, 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"); + } +} |