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use std::marker::PhantomData;
use std::ops::Deref;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::{Arc, Condvar, Mutex};
use std::usize;
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 = (*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 (*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((*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.
(*this).registry
};
let target_worker_index = (*this).target_worker_index;
// NOTE: Once we `set`, the target may proceed and invalidate `this`!
if 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 = (*this).m.lock().unwrap();
*guard = true;
(*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 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 (*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 (*this).kind {
CountLatchKind::Stealing {
ref latch,
ref registry,
worker_index,
} => {
let registry = Arc::clone(registry);
if CoreLatch::set(latch) {
registry.notify_worker_latch_is_set(worker_index);
}
}
CountLatchKind::Blocking { ref latch } => 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) {
L::set((*this).inner);
}
}