miri/concurrency/

sync.rs

use std::cell::RefCell;
use std::collections::VecDeque;
use std::collections::hash_map::Entry;
use std::default::Default;
use std::ops::Not;
use std::rc::Rc;

use rustc_data_structures::fx::FxHashMap;

use super::vector_clock::VClock;
use crate::*;

/// The mutex state.
#[derive(Default, Debug)]
struct Mutex {
    /// The thread that currently owns the lock.
    owner: Option<ThreadId>,
    /// How many times the mutex was locked by the owner.
    lock_count: usize,
    /// The queue of threads waiting for this mutex.
    queue: VecDeque<ThreadId>,
    /// Mutex clock. This tracks the moment of the last unlock.
    clock: VClock,
}

#[derive(Default, Clone, Debug)]
pub struct MutexRef(Rc<RefCell<Mutex>>);

impl MutexRef {
    pub fn new() -> Self {
        Self(Default::default())
    }

    /// Get the id of the thread that currently owns this lock, or `None` if it is not locked.
    pub fn owner(&self) -> Option<ThreadId> {
        self.0.borrow().owner
    }

    pub fn queue_is_empty(&self) -> bool {
        self.0.borrow().queue.is_empty()
    }
}

impl VisitProvenance for MutexRef {
    // Mutex contains no provenance.
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {}
}

/// The read-write lock state.
#[derive(Default, Debug)]
struct RwLock {
    /// The writer thread that currently owns the lock.
    writer: Option<ThreadId>,
    /// The readers that currently own the lock and how many times they acquired
    /// the lock.
    readers: FxHashMap<ThreadId, usize>,
    /// The queue of writer threads waiting for this lock.
    writer_queue: VecDeque<ThreadId>,
    /// The queue of reader threads waiting for this lock.
    reader_queue: VecDeque<ThreadId>,
    /// Data race clock for writers. Tracks the happens-before
    /// ordering between each write access to a rwlock and is updated
    /// after a sequence of concurrent readers to track the happens-
    /// before ordering between the set of previous readers and
    /// the current writer.
    /// Contains the clock of the last thread to release a writer
    /// lock or the joined clock of the set of last threads to release
    /// shared reader locks.
    clock_unlocked: VClock,
    /// Data race clock for readers. This is temporary storage
    /// for the combined happens-before ordering for between all
    /// concurrent readers and the next writer, and the value
    /// is stored to the main data_race variable once all
    /// readers are finished.
    /// Has to be stored separately since reader lock acquires
    /// must load the clock of the last write and must not
    /// add happens-before orderings between shared reader
    /// locks.
    /// This is only relevant when there is an active reader.
    clock_current_readers: VClock,
}

impl RwLock {
    #[inline]
    /// Check if locked.
    fn is_locked(&self) -> bool {
        trace!(
            "rwlock_is_locked: writer is {:?} and there are {} reader threads (some of which could hold multiple read locks)",
            self.writer,
            self.readers.len(),
        );
        self.writer.is_some() || self.readers.is_empty().not()
    }

    /// Check if write locked.
    #[inline]
    fn is_write_locked(&self) -> bool {
        trace!("rwlock_is_write_locked: writer is {:?}", self.writer);
        self.writer.is_some()
    }
}

#[derive(Default, Clone, Debug)]
pub struct RwLockRef(Rc<RefCell<RwLock>>);

impl RwLockRef {
    pub fn new() -> Self {
        Self(Default::default())
    }

    pub fn is_locked(&self) -> bool {
        self.0.borrow().is_locked()
    }

    pub fn is_write_locked(&self) -> bool {
        self.0.borrow().is_write_locked()
    }

    pub fn queue_is_empty(&self) -> bool {
        let inner = self.0.borrow();
        inner.reader_queue.is_empty() && inner.writer_queue.is_empty()
    }
}

impl VisitProvenance for RwLockRef {
    // RwLock contains no provenance.
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {}
}

/// The conditional variable state.
#[derive(Default, Debug)]
struct Condvar {
    waiters: VecDeque<ThreadId>,
    /// Tracks the happens-before relationship
    /// between a cond-var signal and a cond-var
    /// wait during a non-spurious signal event.
    /// Contains the clock of the last thread to
    /// perform a condvar-signal.
    clock: VClock,
}

#[derive(Default, Clone, Debug)]
pub struct CondvarRef(Rc<RefCell<Condvar>>);

impl CondvarRef {
    pub fn new() -> Self {
        Self(Default::default())
    }

    pub fn queue_is_empty(&self) -> bool {
        self.0.borrow().waiters.is_empty()
    }
}

impl VisitProvenance for CondvarRef {
    // Condvar contains no provenance.
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {}
}

/// The semaphore state.
#[derive(Default, Debug)]
struct Semaphore {
    waiters: VecDeque<ThreadId>,
    value: u64,
    /// The joint data race clock of the most recent semaphore increment.
    clock: VClock,
}

#[derive(Default, Clone, Debug)]
pub struct SemaphoreRef(Rc<RefCell<Semaphore>>);

impl SemaphoreRef {
    pub fn new(value: u64) -> Self {
        Self(Rc::new(RefCell::new(Semaphore { value, ..Default::default() })))
    }

    pub fn value(&self) -> u64 {
        self.0.borrow().value
    }

    pub fn queue_is_empty(&self) -> bool {
        self.0.borrow().waiters.is_empty()
    }
}

impl VisitProvenance for SemaphoreRef {
    // Semaphore contains no provenance.
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {}
}

/// The futex state.
#[derive(Default, Debug)]
struct Futex {
    waiters: Vec<FutexWaiter>,
    /// Tracks the happens-before relationship
    /// between a futex-wake and a futex-wait
    /// during a non-spurious wake event.
    /// Contains the clock of the last thread to
    /// perform a futex-wake.
    clock: VClock,
}

#[derive(Default, Clone, Debug)]
pub struct FutexRef(Rc<RefCell<Futex>>);

impl FutexRef {
    pub fn new() -> Self {
        Self(Default::default())
    }

    pub fn waiters(&self) -> usize {
        self.0.borrow().waiters.len()
    }
}

impl VisitProvenance for FutexRef {
    // Futex contains no provenance.
    fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {}
}

/// A thread waiting on a futex.
#[derive(Debug)]
struct FutexWaiter {
    /// The thread that is waiting on this futex.
    thread: ThreadId,
    /// The bitset used by FUTEX_*_BITSET, or u32::MAX for other operations.
    bitset: u32,
}

// Private extension trait for local helper methods
impl<'tcx> EvalContextExtPriv<'tcx> for crate::MiriInterpCx<'tcx> {}
pub(super) trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
    fn condvar_reacquire_mutex(
        &mut self,
        mutex_ref: MutexRef,
        retval: Scalar,
        dest: MPlaceTy<'tcx>,
    ) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();
        if let Some(owner) = mutex_ref.owner() {
            assert_ne!(owner, ecx.active_thread());
            ecx.mutex_enqueue_and_block(mutex_ref, Some((retval, dest)));
        } else {
            // We can have it right now!
            ecx.mutex_lock(&mutex_ref)?;
            // Don't forget to write the return value.
            ecx.write_scalar(retval, &dest)?;
        }
        interp_ok(())
    }
}

// Public interface to synchronization objects. Please note that in most
// cases, the function calls are infallible and it is the client's (shim
// implementation's) responsibility to detect and deal with erroneous
// situations.
impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
    /// Lock by setting the mutex owner and increasing the lock count.
    fn mutex_lock(&self, mutex_ref: &MutexRef) -> InterpResult<'tcx> {
        let ecx = self.eval_context_ref();
        let thread = ecx.active_thread();
        let mut mutex = mutex_ref.0.borrow_mut();
        if let Some(current_owner) = mutex.owner {
            assert_eq!(thread, current_owner, "mutex already locked by another thread");
            assert!(
                mutex.lock_count > 0,
                "invariant violation: lock_count == 0 iff the thread is unlocked"
            );
        } else {
            mutex.owner = Some(thread);
        }
        mutex.lock_count = mutex.lock_count.strict_add(1);
        ecx.acquire_clock(&mutex.clock)?;
        interp_ok(())
    }

    /// Try unlocking by decreasing the lock count and returning the old lock
    /// count. If the lock count reaches 0, release the lock and potentially
    /// give to a new owner. If the lock was not locked by the current thread,
    /// return `None`.
    fn mutex_unlock(&mut self, mutex_ref: &MutexRef) -> InterpResult<'tcx, Option<usize>> {
        let ecx = self.eval_context_mut();
        let mut mutex = mutex_ref.0.borrow_mut();
        interp_ok(if let Some(current_owner) = mutex.owner {
            // Mutex is locked.
            if current_owner != ecx.machine.threads.active_thread() {
                // Only the owner can unlock the mutex.
                return interp_ok(None);
            }
            let old_lock_count = mutex.lock_count;
            mutex.lock_count = old_lock_count.strict_sub(1);
            if mutex.lock_count == 0 {
                mutex.owner = None;
                // The mutex is completely unlocked. Try transferring ownership
                // to another thread.

                ecx.release_clock(|clock| mutex.clock.clone_from(clock))?;
                let thread_id = mutex.queue.pop_front();
                // We need to drop our mutex borrow before unblock_thread
                // because it will be borrowed again in the unblock callback.
                drop(mutex);
                if let Some(thread_id) = thread_id {
                    ecx.unblock_thread(thread_id, BlockReason::Mutex)?;
                }
            }
            Some(old_lock_count)
        } else {
            // Mutex is not locked.
            None
        })
    }

    /// Put the thread into the queue waiting for the mutex.
    ///
    /// Once the Mutex becomes available and if it exists, `retval_dest.0` will
    /// be written to `retval_dest.1`.
    #[inline]
    fn mutex_enqueue_and_block(
        &mut self,
        mutex_ref: MutexRef,
        retval_dest: Option<(Scalar, MPlaceTy<'tcx>)>,
    ) {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut mutex = mutex_ref.0.borrow_mut();
        mutex.queue.push_back(thread);
        assert!(mutex.owner.is_some(), "queuing on unlocked mutex");
        drop(mutex);
        ecx.block_thread(
            BlockReason::Mutex,
            None,
            callback!(
                @capture<'tcx> {
                    mutex_ref: MutexRef,
                    retval_dest: Option<(Scalar, MPlaceTy<'tcx>)>,
                }
                |ecx, unblock: UnblockKind| {
                    assert_eq!(unblock, UnblockKind::Ready);

                    assert!(mutex_ref.owner().is_none());
                    ecx.mutex_lock(&mutex_ref)?;

                    if let Some((retval, dest)) = retval_dest {
                        ecx.write_scalar(retval, &dest)?;
                    }

                    interp_ok(())
                }
            ),
        );
    }

    /// Read-lock the lock by adding the `reader` the list of threads that own
    /// this lock.
    fn rwlock_reader_lock(&self, rwlock_ref: &RwLockRef) -> InterpResult<'tcx> {
        let ecx = self.eval_context_ref();
        let thread = ecx.active_thread();
        trace!("rwlock_reader_lock: now also held (one more time) by {:?}", thread);
        let mut rwlock = rwlock_ref.0.borrow_mut();
        assert!(!rwlock.is_write_locked(), "the lock is write locked");
        let count = rwlock.readers.entry(thread).or_insert(0);
        *count = count.strict_add(1);
        ecx.acquire_clock(&rwlock.clock_unlocked)?;
        interp_ok(())
    }

    /// Try read-unlock the lock for the current threads and potentially give the lock to a new owner.
    /// Returns `true` if succeeded, `false` if this `reader` did not hold the lock.
    fn rwlock_reader_unlock(&mut self, rwlock_ref: &RwLockRef) -> InterpResult<'tcx, bool> {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut rwlock = rwlock_ref.0.borrow_mut();
        match rwlock.readers.entry(thread) {
            Entry::Occupied(mut entry) => {
                let count = entry.get_mut();
                assert!(*count > 0, "rwlock locked with count == 0");
                *count -= 1;
                if *count == 0 {
                    trace!("rwlock_reader_unlock: no longer held by {:?}", thread);
                    entry.remove();
                } else {
                    trace!("rwlock_reader_unlock: held one less time by {:?}", thread);
                }
            }
            Entry::Vacant(_) => return interp_ok(false), // we did not even own this lock
        }
        // Add this to the shared-release clock of all concurrent readers.
        ecx.release_clock(|clock| rwlock.clock_current_readers.join(clock))?;

        // The thread was a reader. If the lock is not held any more, give it to a writer.
        if rwlock.is_locked().not() {
            // All the readers are finished, so set the writer data-race handle to the value
            // of the union of all reader data race handles, since the set of readers
            // happen-before the writers
            let rwlock_ref = &mut *rwlock;
            rwlock_ref.clock_unlocked.clone_from(&rwlock_ref.clock_current_readers);
            // See if there is a thread to unblock.
            if let Some(writer) = rwlock_ref.writer_queue.pop_front() {
                drop(rwlock); // make RefCell available for unblock callback
                ecx.unblock_thread(writer, BlockReason::RwLock)?;
            }
        }
        interp_ok(true)
    }

    /// Put the reader in the queue waiting for the lock and block it.
    /// Once the lock becomes available, `retval` will be written to `dest`.
    #[inline]
    fn rwlock_enqueue_and_block_reader(
        &mut self,
        rwlock_ref: RwLockRef,
        retval: Scalar,
        dest: MPlaceTy<'tcx>,
    ) {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut rwlock = rwlock_ref.0.borrow_mut();
        rwlock.reader_queue.push_back(thread);
        assert!(rwlock.is_write_locked(), "read-queueing on not write locked rwlock");
        drop(rwlock);
        ecx.block_thread(
            BlockReason::RwLock,
            None,
            callback!(
                @capture<'tcx> {
                    rwlock_ref: RwLockRef,
                    retval: Scalar,
                    dest: MPlaceTy<'tcx>,
                }
                |ecx, unblock: UnblockKind| {
                    assert_eq!(unblock, UnblockKind::Ready);
                    ecx.rwlock_reader_lock(&rwlock_ref)?;
                    ecx.write_scalar(retval, &dest)?;
                    interp_ok(())
                }
            ),
        );
    }

    /// Lock by setting the writer that owns the lock.
    #[inline]
    fn rwlock_writer_lock(&self, rwlock_ref: &RwLockRef) -> InterpResult<'tcx> {
        let ecx = self.eval_context_ref();
        let thread = ecx.active_thread();
        trace!("rwlock_writer_lock: now held by {:?}", thread);

        let mut rwlock = rwlock_ref.0.borrow_mut();
        assert!(!rwlock.is_locked(), "the rwlock is already locked");
        rwlock.writer = Some(thread);
        ecx.acquire_clock(&rwlock.clock_unlocked)?;
        interp_ok(())
    }

    /// Try to unlock an rwlock held by the current thread.
    /// Return `false` if it is held by another thread.
    #[inline]
    fn rwlock_writer_unlock(&mut self, rwlock_ref: &RwLockRef) -> InterpResult<'tcx, bool> {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut rwlock = rwlock_ref.0.borrow_mut();
        interp_ok(if let Some(current_writer) = rwlock.writer {
            if current_writer != thread {
                // Only the owner can unlock the rwlock.
                return interp_ok(false);
            }
            rwlock.writer = None;
            trace!("rwlock_writer_unlock: unlocked by {:?}", thread);
            // Record release clock for next lock holder.
            ecx.release_clock(|clock| rwlock.clock_unlocked.clone_from(clock))?;

            // The thread was a writer.
            //
            // We are prioritizing writers here against the readers. As a
            // result, not only readers can starve writers, but also writers can
            // starve readers.
            if let Some(writer) = rwlock.writer_queue.pop_front() {
                drop(rwlock); // make RefCell available for unblock callback
                ecx.unblock_thread(writer, BlockReason::RwLock)?;
            } else {
                // Take the entire read queue and wake them all up.
                let readers = std::mem::take(&mut rwlock.reader_queue);
                drop(rwlock); // make RefCell available for unblock callback
                for reader in readers {
                    ecx.unblock_thread(reader, BlockReason::RwLock)?;
                }
            }
            true
        } else {
            false
        })
    }

    /// Put the writer in the queue waiting for the lock.
    /// Once the lock becomes available, `retval` will be written to `dest`.
    #[inline]
    fn rwlock_enqueue_and_block_writer(
        &mut self,
        rwlock_ref: RwLockRef,
        retval: Scalar,
        dest: MPlaceTy<'tcx>,
    ) {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut rwlock = rwlock_ref.0.borrow_mut();
        rwlock.writer_queue.push_back(thread);
        assert!(rwlock.is_locked(), "write-queueing on unlocked rwlock");
        drop(rwlock);
        ecx.block_thread(
            BlockReason::RwLock,
            None,
            callback!(
                @capture<'tcx> {
                    rwlock_ref: RwLockRef,
                    retval: Scalar,
                    dest: MPlaceTy<'tcx>,
                }
                |ecx, unblock: UnblockKind| {
                    assert_eq!(unblock, UnblockKind::Ready);
                    ecx.rwlock_writer_lock(&rwlock_ref)?;
                    ecx.write_scalar(retval, &dest)?;
                    interp_ok(())
                }
            ),
        );
    }

    /// Release the mutex and let the current thread wait on the given condition variable.
    /// Once it is signaled, the mutex will be acquired and `retval_succ` will be written to `dest`.
    /// If the timeout happens first, `retval_timeout` will be written to `dest`.
    fn condvar_wait(
        &mut self,
        condvar_ref: CondvarRef,
        mutex_ref: MutexRef,
        deadline: Option<Deadline>,
        retval_succ: Scalar,
        retval_timeout: Scalar,
        dest: MPlaceTy<'tcx>,
    ) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();
        if let Some(old_locked_count) = ecx.mutex_unlock(&mutex_ref)? {
            if old_locked_count != 1 {
                throw_unsup_format!(
                    "awaiting a condvar on a mutex acquired multiple times is not supported"
                );
            }
        } else {
            throw_ub_format!(
                "awaiting a condvar on a mutex that is unlocked or owned by a different thread"
            );
        }
        let thread = ecx.active_thread();

        condvar_ref.0.borrow_mut().waiters.push_back(thread);
        ecx.block_thread(
            BlockReason::Condvar,
            deadline,
            callback!(
                @capture<'tcx> {
                    condvar_ref: CondvarRef,
                    mutex_ref: MutexRef,
                    retval_succ: Scalar,
                    retval_timeout: Scalar,
                    dest: MPlaceTy<'tcx>,
                }
                |ecx, unblock: UnblockKind| {
                    match unblock {
                        UnblockKind::Ready => {
                            // The condvar was signaled. Make sure we get the clock for that.
                            ecx.acquire_clock(
                                    &condvar_ref.0.borrow().clock,
                                )?;
                            // Try to acquire the mutex.
                            // The timeout only applies to the first wait (until the signal), not for mutex acquisition.
                            ecx.condvar_reacquire_mutex(mutex_ref, retval_succ, dest)
                        }
                        UnblockKind::TimedOut => {
                            // We have to remove the waiter from the queue again.
                            let thread = ecx.active_thread();
                            let waiters = &mut condvar_ref.0.borrow_mut().waiters;
                            waiters.retain(|waiter| *waiter != thread);
                            // Now get back the lock.
                            ecx.condvar_reacquire_mutex(mutex_ref, retval_timeout, dest)
                        }
                    }
                }
            ),
        );
        interp_ok(())
    }

    /// Wake up some thread (if there is any) sleeping on the conditional
    /// variable. Returns `true` iff any thread was woken up.
    fn condvar_signal(&mut self, condvar_ref: &CondvarRef) -> InterpResult<'tcx, bool> {
        let ecx = self.eval_context_mut();
        let mut condvar = condvar_ref.0.borrow_mut();

        // Each condvar signal happens-before the end of the condvar wake
        ecx.release_clock(|clock| condvar.clock.clone_from(clock))?;
        let Some(waiter) = condvar.waiters.pop_front() else {
            return interp_ok(false);
        };
        drop(condvar);
        ecx.unblock_thread(waiter, BlockReason::Condvar)?;
        interp_ok(true)
    }

    /// Wait for the futex to be signaled, or a timeout. Once the thread is
    /// unblocked, `callback` is called with the unblock reason.
    fn futex_wait(
        &mut self,
        futex_ref: FutexRef,
        bitset: u32,
        deadline: Option<Deadline>,
        callback: DynUnblockCallback<'tcx>,
    ) {
        let ecx = self.eval_context_mut();
        let thread = ecx.active_thread();
        let mut futex = futex_ref.0.borrow_mut();
        let waiters = &mut futex.waiters;
        assert!(waiters.iter().all(|waiter| waiter.thread != thread), "thread is already waiting");
        waiters.push(FutexWaiter { thread, bitset });
        drop(futex);

        ecx.block_thread(
            BlockReason::Futex,
            deadline,
            callback!(
                @capture<'tcx> {
                    futex_ref: FutexRef,
                    callback: DynUnblockCallback<'tcx>,
                }
                |ecx, unblock: UnblockKind| {
                    match unblock {
                        UnblockKind::Ready => {
                            let futex = futex_ref.0.borrow();
                            // Acquire the clock of the futex.
                            ecx.acquire_clock(&futex.clock)?;
                        },
                        UnblockKind::TimedOut => {
                            // Remove the waiter from the futex.
                            let thread = ecx.active_thread();
                            let mut futex = futex_ref.0.borrow_mut();
                            futex.waiters.retain(|waiter| waiter.thread != thread);
                        },
                    }

                    callback.call(ecx, unblock)
                }
            ),
        );
    }

    /// Wake up `count` of the threads in the queue that match any of the bits
    /// in the bitset. Returns how many threads were woken.
    fn futex_wake(
        &mut self,
        futex_ref: &FutexRef,
        bitset: u32,
        count: usize,
    ) -> InterpResult<'tcx, usize> {
        let ecx = self.eval_context_mut();
        let mut futex = futex_ref.0.borrow_mut();

        // Each futex-wake happens-before the end of the futex wait
        ecx.release_clock(|clock| futex.clock.clone_from(clock))?;

        // Remove `count` of the threads in the queue that match any of the bits in the bitset.
        // We collect all of them before unblocking because the unblock callback may access the
        // futex state to retrieve the remaining number of waiters on macOS.
        let wake_threads: Vec<_> =
            futex.waiters.extract_if(.., |w| w.bitset & bitset != 0).take(count).collect();
        drop(futex);

        let woken = wake_threads.len();
        for waiter in wake_threads {
            ecx.unblock_thread(waiter.thread, BlockReason::Futex)?;
        }

        interp_ok(woken)
    }

    /// Tries to decrement the semaphore. Blocks the current thread if that is not possible.
    /// `callback` is invoked after the decrement was successful.
    ///
    /// Once the Mutex becomes available and if it exists, `retval_dest.0` will
    /// be written to `retval_dest.1`.
    fn semaphore_decrement(
        &mut self,
        semaphore_ref: SemaphoreRef,
        deadline: Option<Deadline>,
        callback: DynUnblockCallback<'tcx>,
    ) -> InterpResult<'tcx> {
        let ecx = self.eval_context_mut();

        let mut semaphore = semaphore_ref.0.borrow_mut();
        if let Some(new_value) = semaphore.value.checked_sub(1) {
            semaphore.value = new_value;
            ecx.acquire_clock(&semaphore.clock)?;
            drop(semaphore);
            return callback.call(ecx, UnblockKind::Ready);
        }

        // We have to block.
        semaphore.waiters.push_back(ecx.active_thread());
        drop(semaphore);

        ecx.block_thread(
            BlockReason::Semaphore,
            deadline.clone(),
            callback!(
                @capture<'tcx> {
                    semaphore_ref: SemaphoreRef,
                    deadline: Option<Deadline>,
                    callback: DynUnblockCallback<'tcx>,
                }
                |ecx, unblock: UnblockKind| {
                    match unblock {
                        UnblockKind::TimedOut => {
                            // Remove the waiter.
                            let thread = ecx.active_thread();
                            let mut semaphore = semaphore_ref.0.borrow_mut();
                            semaphore.waiters.retain(|&waiter| waiter != thread);
                            // Finish with callback.
                            return callback.call(ecx, UnblockKind::TimedOut);
                        }
                        UnblockKind::Ready => {
                            // Try again! This time it will definitely work.
                            // (But it's too much of a hassle to assert that.)
                            ecx.semaphore_decrement(semaphore_ref, deadline, callback)
                        }
                    }
                }
            ),
        );
        interp_ok(())
    }

    /// Increment the semaphore.
    /// Returns whether anyone was woken up.
    fn semaphore_increment(&mut self, semaphore_ref: &SemaphoreRef) -> InterpResult<'tcx, bool> {
        let ecx = self.eval_context_mut();

        let mut semaphore = semaphore_ref.0.borrow_mut();
        // If the value is currently 0, we will unlock one thread later.
        let wake_thread =
            (semaphore.value == 0).then_some(()).and_then(|()| semaphore.waiters.pop_front());
        // Increment value.
        let Some(new_value) = semaphore.value.checked_add(1) else {
            throw_unsup_format!("the counter of a semaphore overflowed")
        };
        semaphore.value = new_value;
        // Snapshot vector clock.
        ecx.release_clock(|clock| semaphore.clock.clone_from(clock))?;
        drop(semaphore);

        if let Some(thread) = wake_thread {
            ecx.unblock_thread(thread, BlockReason::Semaphore)?;
        }

        interp_ok(wake_thread.is_some())
    }
}