diff options
Diffstat (limited to 'third_party/rust/tokio/src/park/thread.rs')
| -rw-r--r-- | third_party/rust/tokio/src/park/thread.rs | 346 |
1 files changed, 346 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/park/thread.rs b/third_party/rust/tokio/src/park/thread.rs new file mode 100644 index 0000000000..27ce202439 --- /dev/null +++ b/third_party/rust/tokio/src/park/thread.rs @@ -0,0 +1,346 @@ +#![cfg_attr(not(feature = "full"), allow(dead_code))] + +use crate::loom::sync::atomic::AtomicUsize; +use crate::loom::sync::{Arc, Condvar, Mutex}; +use crate::park::{Park, Unpark}; + +use std::sync::atomic::Ordering::SeqCst; +use std::time::Duration; + +#[derive(Debug)] +pub(crate) struct ParkThread { + inner: Arc<Inner>, +} + +pub(crate) type ParkError = (); + +/// Unblocks a thread that was blocked by `ParkThread`. +#[derive(Clone, Debug)] +pub(crate) struct UnparkThread { + inner: Arc<Inner>, +} + +#[derive(Debug)] +struct Inner { + state: AtomicUsize, + mutex: Mutex<()>, + condvar: Condvar, +} + +const EMPTY: usize = 0; +const PARKED: usize = 1; +const NOTIFIED: usize = 2; + +thread_local! { + static CURRENT_PARKER: ParkThread = ParkThread::new(); +} + +// ==== impl ParkThread ==== + +impl ParkThread { + pub(crate) fn new() -> Self { + Self { + inner: Arc::new(Inner { + state: AtomicUsize::new(EMPTY), + mutex: Mutex::new(()), + condvar: Condvar::new(), + }), + } + } +} + +impl Park for ParkThread { + type Unpark = UnparkThread; + type Error = ParkError; + + fn unpark(&self) -> Self::Unpark { + let inner = self.inner.clone(); + UnparkThread { inner } + } + + fn park(&mut self) -> Result<(), Self::Error> { + self.inner.park(); + Ok(()) + } + + fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> { + self.inner.park_timeout(duration); + Ok(()) + } + + fn shutdown(&mut self) { + self.inner.shutdown(); + } +} + +// ==== impl Inner ==== + +impl Inner { + /// Parks the current thread for at most `dur`. + fn park(&self) { + // If we were previously notified then we consume this notification and + // return quickly. + if self + .state + .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) + .is_ok() + { + return; + } + + // Otherwise we need to coordinate going to sleep + let mut m = self.mutex.lock(); + + match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) { + Ok(_) => {} + Err(NOTIFIED) => { + // We must read here, even though we know it will be `NOTIFIED`. + // This is because `unpark` may have been called again since we read + // `NOTIFIED` in the `compare_exchange` above. We must perform an + // acquire operation that synchronizes with that `unpark` to observe + // any writes it made before the call to unpark. To do that we must + // read from the write it made to `state`. + let old = self.state.swap(EMPTY, SeqCst); + debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly"); + + return; + } + Err(actual) => panic!("inconsistent park state; actual = {}", actual), + } + + loop { + m = self.condvar.wait(m).unwrap(); + + if self + .state + .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) + .is_ok() + { + // got a notification + return; + } + + // spurious wakeup, go back to sleep + } + } + + fn park_timeout(&self, dur: Duration) { + // Like `park` above we have a fast path for an already-notified thread, + // and afterwards we start coordinating for a sleep. Return quickly. + if self + .state + .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) + .is_ok() + { + return; + } + + if dur == Duration::from_millis(0) { + return; + } + + let m = self.mutex.lock(); + + match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) { + Ok(_) => {} + Err(NOTIFIED) => { + // We must read again here, see `park`. + let old = self.state.swap(EMPTY, SeqCst); + debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly"); + + return; + } + Err(actual) => panic!("inconsistent park_timeout state; actual = {}", actual), + } + + // Wait with a timeout, and if we spuriously wake up or otherwise wake up + // from a notification, we just want to unconditionally set the state back to + // empty, either consuming a notification or un-flagging ourselves as + // parked. + let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap(); + + match self.state.swap(EMPTY, SeqCst) { + NOTIFIED => {} // got a notification, hurray! + PARKED => {} // no notification, alas + n => panic!("inconsistent park_timeout state: {}", n), + } + } + + fn unpark(&self) { + // To ensure the unparked thread will observe any writes we made before + // this call, we must perform a release operation that `park` can + // synchronize with. To do that we must write `NOTIFIED` even if `state` + // is already `NOTIFIED`. That is why this must be a swap rather than a + // compare-and-swap that returns if it reads `NOTIFIED` on failure. + match self.state.swap(NOTIFIED, SeqCst) { + EMPTY => return, // no one was waiting + NOTIFIED => return, // already unparked + PARKED => {} // gotta go wake someone up + _ => panic!("inconsistent state in unpark"), + } + + // There is a period between when the parked thread sets `state` to + // `PARKED` (or last checked `state` in the case of a spurious wake + // up) and when it actually waits on `cvar`. If we were to notify + // during this period it would be ignored and then when the parked + // thread went to sleep it would never wake up. Fortunately, it has + // `lock` locked at this stage so we can acquire `lock` to wait until + // it is ready to receive the notification. + // + // Releasing `lock` before the call to `notify_one` means that when the + // parked thread wakes it doesn't get woken only to have to wait for us + // to release `lock`. + drop(self.mutex.lock()); + + self.condvar.notify_one() + } + + fn shutdown(&self) { + self.condvar.notify_all(); + } +} + +impl Default for ParkThread { + fn default() -> Self { + Self::new() + } +} + +// ===== impl UnparkThread ===== + +impl Unpark for UnparkThread { + fn unpark(&self) { + self.inner.unpark(); + } +} + +use std::future::Future; +use std::marker::PhantomData; +use std::mem; +use std::rc::Rc; +use std::task::{RawWaker, RawWakerVTable, Waker}; + +/// Blocks the current thread using a condition variable. +#[derive(Debug)] +pub(crate) struct CachedParkThread { + _anchor: PhantomData<Rc<()>>, +} + +impl CachedParkThread { + /// Creates a new `ParkThread` handle for the current thread. + /// + /// This type cannot be moved to other threads, so it should be created on + /// the thread that the caller intends to park. + pub(crate) fn new() -> CachedParkThread { + CachedParkThread { + _anchor: PhantomData, + } + } + + pub(crate) fn get_unpark(&self) -> Result<UnparkThread, ParkError> { + self.with_current(|park_thread| park_thread.unpark()) + } + + /// Gets a reference to the `ParkThread` handle for this thread. + fn with_current<F, R>(&self, f: F) -> Result<R, ParkError> + where + F: FnOnce(&ParkThread) -> R, + { + CURRENT_PARKER.try_with(|inner| f(inner)).map_err(|_| ()) + } + + pub(crate) fn block_on<F: Future>(&mut self, f: F) -> Result<F::Output, ParkError> { + use std::task::Context; + use std::task::Poll::Ready; + + // `get_unpark()` should not return a Result + let waker = self.get_unpark()?.into_waker(); + let mut cx = Context::from_waker(&waker); + + pin!(f); + + loop { + if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) { + return Ok(v); + } + + self.park()?; + } + } +} + +impl Park for CachedParkThread { + type Unpark = UnparkThread; + type Error = ParkError; + + fn unpark(&self) -> Self::Unpark { + self.get_unpark().unwrap() + } + + fn park(&mut self) -> Result<(), Self::Error> { + self.with_current(|park_thread| park_thread.inner.park())?; + Ok(()) + } + + fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> { + self.with_current(|park_thread| park_thread.inner.park_timeout(duration))?; + Ok(()) + } + + fn shutdown(&mut self) { + let _ = self.with_current(|park_thread| park_thread.inner.shutdown()); + } +} + +impl UnparkThread { + pub(crate) fn into_waker(self) -> Waker { + unsafe { + let raw = unparker_to_raw_waker(self.inner); + Waker::from_raw(raw) + } + } +} + +impl Inner { + #[allow(clippy::wrong_self_convention)] + fn into_raw(this: Arc<Inner>) -> *const () { + Arc::into_raw(this) as *const () + } + + unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> { + Arc::from_raw(ptr as *const Inner) + } +} + +unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker { + RawWaker::new( + Inner::into_raw(unparker), + &RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker), + ) +} + +unsafe fn clone(raw: *const ()) -> RawWaker { + let unparker = Inner::from_raw(raw); + + // Increment the ref count + mem::forget(unparker.clone()); + + unparker_to_raw_waker(unparker) +} + +unsafe fn drop_waker(raw: *const ()) { + let _ = Inner::from_raw(raw); +} + +unsafe fn wake(raw: *const ()) { + let unparker = Inner::from_raw(raw); + unparker.unpark(); +} + +unsafe fn wake_by_ref(raw: *const ()) { + let unparker = Inner::from_raw(raw); + unparker.unpark(); + + // We don't actually own a reference to the unparker + mem::forget(unparker); +} |