portable-atomic 1.14.0

// SPDX-License-Identifier: Apache-2.0 OR MIT

// Adapted from https://github.com/crossbeam-rs/crossbeam/blob/crossbeam-utils-0.8.21/crossbeam-utils/src/atomic/seq_lock.rs
// and https://github.com/crossbeam-rs/crossbeam/blob/crossbeam-utils-0.8.21/crossbeam-utils/src/atomic/seq_lock_wide.rs.

use core::{mem::ManuallyDrop, sync::atomic::Ordering};

use super::utils::Backoff;
#[cfg(portable_atomic_unsafe_assume_privileged)]
use crate::imp::interrupt::arch as interrupt;

const LOCKED: State = 1;

impl SeqLock {
    /// Grabs the lock for writing.
    #[inline]
    pub(super) fn write(&self) -> SeqLockWriteGuard<'_> {
        // Get current interrupt state and disable interrupts when the user
        // explicitly declares that privileged instructions are available.
        #[cfg(portable_atomic_unsafe_assume_privileged)]
        let interrupt_state = interrupt::disable();

        let mut backoff = Backoff::new();
        loop {
            let previous = self.state.swap(LOCKED, Ordering::Acquire);

            if previous != LOCKED {
                // To synchronize with the acquire fence in `validate_read` via any modification to
                // the data at the critical section of `previous` (seq_lock.rs) or `(state_hi, previous)`
                // (seq_lock_wide.rs).
                crate::fence(Ordering::Release);

                return SeqLockWriteGuard {
                    lock: self,
                    state: previous,
                    #[cfg(portable_atomic_unsafe_assume_privileged)]
                    interrupt_state,
                };
            }

            while self.state.load(Ordering::Relaxed) == LOCKED {
                backoff.snooze();
            }
        }
    }
}

/// An RAII guard that releases the lock and increments the stamp when dropped.
#[must_use]
pub(crate) struct SeqLockWriteGuard<'a> {
    /// The parent lock.
    lock: &'a SeqLock,

    /// The stamp before locking.
    state: State,

    /// The interrupt state before disabling.
    #[cfg(portable_atomic_unsafe_assume_privileged)]
    interrupt_state: interrupt::State,
}

impl SeqLockWriteGuard<'_> {
    /// Releases the lock without incrementing the stamp.
    #[inline]
    pub(crate) 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);

        // Restore interrupt state.
        // SAFETY: the state was retrieved by the previous `disable`.
        #[cfg(portable_atomic_unsafe_assume_privileged)]
        unsafe {
            interrupt::restore(this.interrupt_state);
        }
    }
}

impl Drop for SeqLockWriteGuard<'_> {
    #[inline]
    fn drop(&mut self) {
        let state = self.next_stamp();

        // Release the lock and increment the stamp.
        //
        // Release ordering for synchronizing with `optimistic_read`.
        self.lock.state.store(state, Ordering::Release);

        // Restore interrupt state.
        // SAFETY: the state was retrieved by the previous `disable`.
        #[cfg(portable_atomic_unsafe_assume_privileged)]
        unsafe {
            interrupt::restore(self.interrupt_state);
        }
    }
}

#[cfg(test)]
#[allow(clippy::items_after_test_module)]
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.optimistic_read().is_none());
        }
        assert!(!lock.validate_read(before));
        let after = lock.optimistic_read().unwrap();
        assert_ne!(before, after);
    }

    #[test]
    fn test_wrap() {
        let mut lock = SeqLock::new();
        let zero = lock.optimistic_read().unwrap();
        *lock.state.get_mut() = 0xFFFF_FFFE; // u32::MAX & !1
        let before = lock.optimistic_read().unwrap();
        assert!(lock.validate_read(before));
        {
            let _guard = lock.write();
            assert!(lock.optimistic_read().is_none());
        }
        assert!(!lock.validate_read(before));
        let after = lock.optimistic_read().unwrap();
        assert_ne!(before, after);
        assert_ne!(after, zero);
        #[allow(clippy::identity_op)]
        {
            assert_eq!(*lock.state.get_mut() & 0xFFFF_FFFF, 0);
        }
    }

    #[test]
    fn test_abort() {
        let lock = SeqLock::new();
        let before = lock.optimistic_read().unwrap();
        {
            let guard = lock.write();
            assert!(lock.optimistic_read().is_none());
            guard.abort();
        }
        let after = lock.optimistic_read().unwrap();
        assert_eq!(before, after, "aborted write does not update the stamp");
    }
}