rustc_thread_pool/

latch.rs

use std::marker::PhantomData;
use std::ops::Deref;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::{Arc, Condvar, Mutex};

use crate::registry::{Registry, WorkerThread};

/// We define various kinds of latches, which are all a primitive signaling
/// mechanism. A latch starts as false. Eventually someone calls `set()` and
/// it becomes true. You can test if it has been set by calling `probe()`.
///
/// Some kinds of latches, but not all, support a `wait()` operation
/// that will wait until the latch is set, blocking efficiently. That
/// is not part of the trait since it is not possibly to do with all
/// latches.
///
/// The intention is that `set()` is called once, but `probe()` may be
/// called any number of times. Once `probe()` returns true, the memory
/// effects that occurred before `set()` become visible.
///
/// It'd probably be better to refactor the API into two paired types,
/// but that's a bit of work, and this is not a public API.
///
/// ## Memory ordering
///
/// Latches need to guarantee two things:
///
/// - Once `probe()` returns true, all memory effects from the `set()`
///   are visible (in other words, the set should synchronize-with
///   the probe).
/// - Once `set()` occurs, the next `probe()` *will* observe it. This
///   typically requires a seq-cst ordering. See [the "tickle-then-get-sleepy" scenario in the sleep
///   README](/src/sleep/README.md#tickle-then-get-sleepy) for details.
pub(super) trait Latch {
    /// Set the latch, signalling others.
    ///
    /// # WARNING
    ///
    /// Setting a latch triggers other threads to wake up and (in some
    /// cases) complete. This may, in turn, cause memory to be
    /// deallocated and so forth. One must be very careful about this,
    /// and it's typically better to read all the fields you will need
    /// to access *before* a latch is set!
    ///
    /// This function operates on `*const Self` instead of `&self` to allow it
    /// to become dangling during this call. The caller must ensure that the
    /// pointer is valid upon entry, and not invalidated during the call by any
    /// actions other than `set` itself.
    unsafe fn set(this: *const Self);
}

pub(super) trait AsCoreLatch {
    fn as_core_latch(&self) -> &CoreLatch;
}

/// Latch is not set, owning thread is awake
const UNSET: usize = 0;

/// Latch is not set, owning thread is going to sleep on this latch
/// (but has not yet fallen asleep).
const SLEEPY: usize = 1;

/// Latch is not set, owning thread is asleep on this latch and
/// must be awoken.
const SLEEPING: usize = 2;

/// Latch is set.
const SET: usize = 3;

/// Spin latches are the simplest, most efficient kind, but they do
/// not support a `wait()` operation. They just have a boolean flag
/// that becomes true when `set()` is called.
#[derive(Debug)]
pub(super) struct CoreLatch {
    state: AtomicUsize,
}

impl CoreLatch {
    #[inline]
    fn new() -> Self {
        Self { state: AtomicUsize::new(0) }
    }

    /// Invoked by owning thread as it prepares to sleep. Returns true
    /// if the owning thread may proceed to fall asleep, false if the
    /// latch was set in the meantime.
    #[inline]
    pub(super) fn get_sleepy(&self) -> bool {
        self.state.compare_exchange(UNSET, SLEEPY, Ordering::SeqCst, Ordering::Relaxed).is_ok()
    }

    /// Invoked by owning thread as it falls asleep sleep. Returns
    /// true if the owning thread should block, or false if the latch
    /// was set in the meantime.
    #[inline]
    pub(super) fn fall_asleep(&self) -> bool {
        self.state.compare_exchange(SLEEPY, SLEEPING, Ordering::SeqCst, Ordering::Relaxed).is_ok()
    }

    /// Invoked by owning thread as it falls asleep sleep. Returns
    /// true if the owning thread should block, or false if the latch
    /// was set in the meantime.
    #[inline]
    pub(super) fn wake_up(&self) {
        if !self.probe() {
            let _ =
                self.state.compare_exchange(SLEEPING, UNSET, Ordering::SeqCst, Ordering::Relaxed);
        }
    }

    /// Set the latch. If this returns true, the owning thread was sleeping
    /// and must be awoken.
    ///
    /// This is private because, typically, setting a latch involves
    /// doing some wakeups; those are encapsulated in the surrounding
    /// latch code.
    #[inline]
    unsafe fn set(this: *const Self) -> bool {
        let old_state = unsafe { (*this).state.swap(SET, Ordering::AcqRel) };
        old_state == SLEEPING
    }

    /// Test if this latch has been set.
    #[inline]
    pub(super) fn probe(&self) -> bool {
        self.state.load(Ordering::Acquire) == SET
    }
}

impl AsCoreLatch for CoreLatch {
    #[inline]
    fn as_core_latch(&self) -> &CoreLatch {
        self
    }
}

/// Spin latches are the simplest, most efficient kind, but they do
/// not support a `wait()` operation. They just have a boolean flag
/// that becomes true when `set()` is called.
pub(super) struct SpinLatch<'r> {
    core_latch: CoreLatch,
    registry: &'r Arc<Registry>,
    target_worker_index: usize,
    cross: bool,
}

impl<'r> SpinLatch<'r> {
    /// Creates a new spin latch that is owned by `thread`. This means
    /// that `thread` is the only thread that should be blocking on
    /// this latch -- it also means that when the latch is set, we
    /// will wake `thread` if it is sleeping.
    #[inline]
    pub(super) fn new(thread: &'r WorkerThread) -> SpinLatch<'r> {
        SpinLatch {
            core_latch: CoreLatch::new(),
            registry: thread.registry(),
            target_worker_index: thread.index(),
            cross: false,
        }
    }

    /// Creates a new spin latch for cross-threadpool blocking. Notably, we
    /// need to make sure the registry is kept alive after setting, so we can
    /// safely call the notification.
    #[inline]
    pub(super) fn cross(thread: &'r WorkerThread) -> SpinLatch<'r> {
        SpinLatch { cross: true, ..SpinLatch::new(thread) }
    }

    #[inline]
    pub(super) fn probe(&self) -> bool {
        self.core_latch.probe()
    }
}

impl<'r> AsCoreLatch for SpinLatch<'r> {
    #[inline]
    fn as_core_latch(&self) -> &CoreLatch {
        &self.core_latch
    }
}

impl<'r> Latch for SpinLatch<'r> {
    #[inline]
    unsafe fn set(this: *const Self) {
        let cross_registry;

        let registry: &Registry = if unsafe { (*this).cross } {
            // Ensure the registry stays alive while we notify it.
            // Otherwise, it would be possible that we set the spin
            // latch and the other thread sees it and exits, causing
            // the registry to be deallocated, all before we get a
            // chance to invoke `registry.notify_worker_latch_is_set`.
            cross_registry = Arc::clone(unsafe { (*this).registry });
            &cross_registry
        } else {
            // If this is not a "cross-registry" spin-latch, then the
            // thread which is performing `set` is itself ensuring
            // that the registry stays alive. However, that doesn't
            // include this *particular* `Arc` handle if the waiting
            // thread then exits, so we must completely dereference it.
            unsafe { (*this).registry }
        };
        let target_worker_index = unsafe { (*this).target_worker_index };

        // NOTE: Once we `set`, the target may proceed and invalidate `this`!
        if unsafe { CoreLatch::set(&(*this).core_latch) } {
            // Subtle: at this point, we can no longer read from
            // `self`, because the thread owning this spin latch may
            // have awoken and deallocated the latch. Therefore, we
            // only use fields whose values we already read.
            registry.notify_worker_latch_is_set(target_worker_index);
        }
    }
}

/// A Latch starts as false and eventually becomes true. You can block
/// until it becomes true.
#[derive(Debug)]
pub(super) struct LockLatch {
    m: Mutex<bool>,
    v: Condvar,
}

impl LockLatch {
    #[inline]
    pub(super) fn new() -> LockLatch {
        LockLatch { m: Mutex::new(false), v: Condvar::new() }
    }

    /// Block until latch is set, then resets this lock latch so it can be reused again.
    pub(super) fn wait_and_reset(&self) {
        let mut guard = self.m.lock().unwrap();
        while !*guard {
            guard = self.v.wait(guard).unwrap();
        }
        *guard = false;
    }

    /// Block until latch is set.
    pub(super) fn wait(&self) {
        let mut guard = self.m.lock().unwrap();
        while !*guard {
            guard = self.v.wait(guard).unwrap();
        }
    }
}

impl Latch for LockLatch {
    #[inline]
    unsafe fn set(this: *const Self) {
        let mut guard = unsafe { (*this).m.lock().unwrap() };
        *guard = true;
        unsafe { (*this).v.notify_all() };
    }
}

/// Once latches are used to implement one-time blocking, primarily
/// for the termination flag of the threads in the pool.
///
/// Note: like a `SpinLatch`, once-latches are always associated with
/// some registry that is probing them, which must be tickled when
/// they are set. *Unlike* a `SpinLatch`, they don't themselves hold a
/// reference to that registry. This is because in some cases the
/// registry owns the once-latch, and that would create a cycle. So a
/// `OnceLatch` must be given a reference to its owning registry when
/// it is set. For this reason, it does not implement the `Latch`
/// trait (but it doesn't have to, as it is not used in those generic
/// contexts).
#[derive(Debug)]
pub(super) struct OnceLatch {
    core_latch: CoreLatch,
}

impl OnceLatch {
    #[inline]
    pub(super) fn new() -> OnceLatch {
        Self { core_latch: CoreLatch::new() }
    }

    /// Set the latch, then tickle the specific worker thread,
    /// which should be the one that owns this latch.
    #[inline]
    pub(super) unsafe fn set_and_tickle_one(
        this: *const Self,
        registry: &Registry,
        target_worker_index: usize,
    ) {
        if unsafe { CoreLatch::set(&(*this).core_latch) } {
            registry.notify_worker_latch_is_set(target_worker_index);
        }
    }
}

impl AsCoreLatch for OnceLatch {
    #[inline]
    fn as_core_latch(&self) -> &CoreLatch {
        &self.core_latch
    }
}

/// Counting latches are used to implement scopes. They track a
/// counter. Unlike other latches, calling `set()` does not
/// necessarily make the latch be considered `set()`; instead, it just
/// decrements the counter. The latch is only "set" (in the sense that
/// `probe()` returns true) once the counter reaches zero.
#[derive(Debug)]
pub(super) struct CountLatch {
    counter: AtomicUsize,
    kind: CountLatchKind,
}

enum CountLatchKind {
    /// A latch for scopes created on a rayon thread which will participate in work-
    /// stealing while it waits for completion. This thread is not necessarily part
    /// of the same registry as the scope itself!
    Stealing {
        latch: CoreLatch,
        /// If a worker thread in registry A calls `in_place_scope` on a ThreadPool
        /// with registry B, when a job completes in a thread of registry B, we may
        /// need to call `notify_worker_latch_is_set()` to wake the thread in registry A.
        /// That means we need a reference to registry A (since at that point we will
        /// only have a reference to registry B), so we stash it here.
        registry: Arc<Registry>,
        /// The index of the worker to wake in `registry`
        worker_index: usize,
    },

    /// A latch for scopes created on a non-rayon thread which will block to wait.
    Blocking { latch: LockLatch },
}

impl std::fmt::Debug for CountLatchKind {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            CountLatchKind::Stealing { latch, .. } => {
                f.debug_tuple("Stealing").field(latch).finish()
            }
            CountLatchKind::Blocking { latch, .. } => {
                f.debug_tuple("Blocking").field(latch).finish()
            }
        }
    }
}

impl CountLatch {
    pub(super) fn new(owner: Option<&WorkerThread>) -> Self {
        Self::with_count(1, owner)
    }

    pub(super) fn with_count(count: usize, owner: Option<&WorkerThread>) -> Self {
        Self {
            counter: AtomicUsize::new(count),
            kind: match owner {
                Some(owner) => CountLatchKind::Stealing {
                    latch: CoreLatch::new(),
                    registry: Arc::clone(owner.registry()),
                    worker_index: owner.index(),
                },
                None => CountLatchKind::Blocking { latch: LockLatch::new() },
            },
        }
    }

    #[inline]
    pub(super) fn increment(&self) {
        let old_counter = self.counter.fetch_add(1, Ordering::Relaxed);
        debug_assert!(old_counter != 0);
    }

    pub(super) fn wait(&self, owner: Option<&WorkerThread>) {
        match &self.kind {
            CountLatchKind::Stealing { latch, registry, worker_index } => unsafe {
                let owner = owner.expect("owner thread");
                debug_assert_eq!(registry.id(), owner.registry().id());
                debug_assert_eq!(*worker_index, owner.index());
                owner.wait_until(latch);
            },
            CountLatchKind::Blocking { latch } => latch.wait(),
        }
    }
}

impl Latch for CountLatch {
    #[inline]
    unsafe fn set(this: *const Self) {
        if unsafe { (*this).counter.fetch_sub(1, Ordering::SeqCst) == 1 } {
            // NOTE: Once we call `set` on the internal `latch`,
            // the target may proceed and invalidate `this`!
            match unsafe { &(*this).kind } {
                CountLatchKind::Stealing { latch, registry, worker_index } => {
                    let registry = Arc::clone(registry);
                    if unsafe { CoreLatch::set(latch) } {
                        registry.notify_worker_latch_is_set(*worker_index);
                    }
                }
                CountLatchKind::Blocking { latch } => unsafe { LockLatch::set(latch) },
            }
        }
    }
}

/// `&L` without any implication of `dereferenceable` for `Latch::set`
pub(super) struct LatchRef<'a, L> {
    inner: *const L,
    marker: PhantomData<&'a L>,
}

impl<L> LatchRef<'_, L> {
    pub(super) fn new(inner: &L) -> LatchRef<'_, L> {
        LatchRef { inner, marker: PhantomData }
    }
}

unsafe impl<L: Sync> Sync for LatchRef<'_, L> {}

impl<L> Deref for LatchRef<'_, L> {
    type Target = L;

    fn deref(&self) -> &L {
        // SAFETY: if we have &self, the inner latch is still alive
        unsafe { &*self.inner }
    }
}

impl<L: Latch> Latch for LatchRef<'_, L> {
    #[inline]
    unsafe fn set(this: *const Self) {
        unsafe { L::set((*this).inner) };
    }
}