1 files changed, 147 insertions, 0 deletions
diff --git a/vendor/portable-atomic/src/imp/fallback/seq_lock.rs b/vendor/portable-atomic/src/imp/fallback/seq_lock.rs
new file mode 100644
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+++ b/vendor/portable-atomic/src/imp/fallback/seq_lock.rs
@@ -0,0 +1,147 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+// Adapted from https://github.com/crossbeam-rs/crossbeam/blob/crossbeam-utils-0.8.7/crossbeam-utils/src/atomic/seq_lock.rs.
+
+use core::{
+    mem::ManuallyDrop,
+    sync::atomic::{self, Ordering},
+};
+
+use super::utils::Backoff;
+
+// See mod.rs for details.
+#[cfg(any(target_pointer_width = "16", target_pointer_width = "32"))]
+pub(super) use core::sync::atomic::AtomicU64 as AtomicStamp;
+#[cfg(not(any(target_pointer_width = "16", target_pointer_width = "32")))]
+pub(super) use core::sync::atomic::AtomicUsize as AtomicStamp;
+#[cfg(not(any(target_pointer_width = "16", target_pointer_width = "32")))]
+pub(super) type Stamp = usize;
+#[cfg(any(target_pointer_width = "16", target_pointer_width = "32"))]
+pub(super) type Stamp = u64;
+
+// See mod.rs for details.
+pub(super) type AtomicChunk = AtomicStamp;
+pub(super) type Chunk = Stamp;
+
+/// A simple stamped lock.
+pub(super) 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: AtomicStamp,
+}
+
+impl SeqLock {
+    #[inline]
+    pub(super) const fn new() -> Self {
+        Self { state: AtomicStamp::new(0) }
+    }
+
+    /// If not locked, returns the current stamp.
+    ///
+    /// This method should be called before optimistic reads.
+    #[inline]
+    pub(super) fn optimistic_read(&self) -> Option<Stamp> {
+        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(super) fn validate_read(&self, stamp: Stamp) -> bool {
+        atomic::fence(Ordering::Acquire);
+        self.state.load(Ordering::Relaxed) == stamp
+    }
+
+    /// Grabs the lock for writing.
+    #[inline]
+    pub(super) fn write(&self) -> SeqLockWriteGuard<'_> {
+        let mut 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 };
+            }
+
+            while self.state.load(Ordering::Relaxed) == 1 {
+                backoff.snooze();
+            }
+        }
+    }
+}
+
+/// An RAII guard that releases the lock and increments the stamp when dropped.
+#[must_use]
+pub(super) struct SeqLockWriteGuard<'a> {
+    /// The parent lock.
+    lock: &'a SeqLock,
+
+    /// The stamp before locking.
+    state: Stamp,
+}
+
+impl SeqLockWriteGuard<'_> {
+    /// Releases the lock without incrementing the stamp.
+    #[inline]
+    pub(super) fn abort(self) {
+        // We specifically don't want to call drop(), since that's
+        // what increments the stamp.
+        let this = ManuallyDrop::new(self);
+
+        // Restore the stamp.
+        //
+        // Release ordering for synchronizing with `optimistic_read`.
+        this.lock.state.store(this.state, Ordering::Release);
+    }
+}
+
+impl Drop for SeqLockWriteGuard<'_> {
+    #[inline]
+    fn drop(&mut self) {
+        // Release the lock and increment the stamp.
+        //
+        // Release ordering for synchronizing with `optimistic_read`.
+        self.lock.state.store(self.state.wrapping_add(2), Ordering::Release);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::SeqLock;
+
+    #[test]
+    fn smoke() {
+        let lock = SeqLock::new();
+        let before = lock.optimistic_read().unwrap();
+        assert!(lock.validate_read(before));
+        {
+            let _guard = lock.write();
+        }
+        assert!(!lock.validate_read(before));
+        let after = lock.optimistic_read().unwrap();
+        assert_ne!(before, after);
+    }
+
+    #[test]
+    fn test_abort() {
+        let lock = SeqLock::new();
+        let before = lock.optimistic_read().unwrap();
+        {
+            let guard = lock.write();
+            guard.abort();
+        }
+        let after = lock.optimistic_read().unwrap();
+        assert_eq!(before, after, "aborted write does not update the stamp");
+    }
+}