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authorValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
committerValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
commita990de90fe41456a23e58bd087d2f107d321f3a1 (patch)
tree15afc392522a9e85dc3332235e311b7d39352ea9 /vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs
parent3d48cd3f81164bbfc1a755dc1d4a9a02f98c8ddd (diff)
downloadfparkan-a990de90fe41456a23e58bd087d2f107d321f3a1.tar.xz
fparkan-a990de90fe41456a23e58bd087d2f107d321f3a1.zip
Deleted vendor folder
Diffstat (limited to 'vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs')
-rw-r--r--vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs155
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diff --git a/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs b/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs
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--- a/vendor/crossbeam-utils/src/atomic/seq_lock_wide.rs
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-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");
- }
-}