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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");
}
}