tokio/runtime/time/

entry.rs

//! Timer state structures.
//!
//! This module contains the heart of the intrusive timer implementation, and as
//! such the structures inside are full of tricky concurrency and unsafe code.
//!
//! # Ground rules
//!
//! The heart of the timer implementation here is the [`TimerShared`] structure,
//! shared between the [`TimerEntry`] and the driver. Generally, we permit access
//! to [`TimerShared`] ONLY via either 1) a mutable reference to [`TimerEntry`] or
//! 2) a held driver lock.
//!
//! It follows from this that any changes made while holding BOTH 1 and 2 will
//! be reliably visible, regardless of ordering. This is because of the `acq/rel`
//! fences on the driver lock ensuring ordering with 2, and rust mutable
//! reference rules for 1 (a mutable reference to an object can't be passed
//! between threads without an `acq/rel` barrier, and same-thread we have local
//! happens-before ordering).
//!
//! # State field
//!
//! Each timer has a state field associated with it. This field contains either
//! the current scheduled time, or a special flag value indicating its state.
//! This state can either indicate that the timer is on the 'pending' queue (and
//! thus will be fired with an `Ok(())` result soon) or that it has already been
//! fired/deregistered.
//!
//! This single state field allows for code that is firing the timer to
//! synchronize with any racing `reset` calls reliably.
//!
//! # Registered vs true timeouts
//!
//! To allow for the use case of a timeout that is periodically reset before
//! expiration to be as lightweight as possible, we support optimistically
//! lock-free timer resets, in the case where a timer is rescheduled to a later
//! point than it was originally scheduled for.
//!
//! This is accomplished by lazily rescheduling timers. That is, we update the
//! state field with the true expiration of the timer from the holder of
//! the [`TimerEntry`]. When the driver services timers (ie, whenever it's
//! walking lists of timers), it checks this "true when" value, and reschedules
//! based on it.
//!
//! We do, however, also need to track what the expiration time was when we
//! originally registered the timer; this is used to locate the right linked
//! list when the timer is being cancelled.
//! This is referred to as the `registered_when` internally.
//!
//! There is of course a race condition between timer reset and timer
//! expiration. If the driver fails to observe the updated expiration time, it
//! could trigger expiration of the timer too early. However, because
//! [`mark_pending`][mark_pending] performs a compare-and-swap, it will identify this race and
//! refuse to mark the timer as pending.
//!
//! [mark_pending]: TimerHandle::mark_pending

use crate::loom::cell::UnsafeCell;
use crate::loom::sync::atomic::AtomicU64;
use crate::loom::sync::atomic::Ordering;

use crate::runtime::scheduler;
use crate::sync::AtomicWaker;
use crate::util::linked_list;

use pin_project_lite::pin_project;
use std::task::{Context, Poll, Waker};
use std::{marker::PhantomPinned, pin::Pin, ptr::NonNull};

type TimerResult = Result<(), crate::time::error::Error>;

pub(in crate::runtime::time) const STATE_DEREGISTERED: u64 = u64::MAX;
const STATE_PENDING_FIRE: u64 = STATE_DEREGISTERED - 1;
const STATE_MIN_VALUE: u64 = STATE_PENDING_FIRE;
/// The largest safe integer to use for ticks.
///
/// This value should be updated if any other signal values are added above.
pub(super) const MAX_SAFE_MILLIS_DURATION: u64 = STATE_MIN_VALUE - 1;

/// This structure holds the current shared state of the timer - its scheduled
/// time (if registered), or otherwise the result of the timer completing, as
/// well as the registered waker.
///
/// Generally, the `StateCell` is only permitted to be accessed from two contexts:
/// Either a thread holding the corresponding `&mut TimerEntry`, or a thread
/// holding the timer driver lock. The write actions on the `StateCell` amount to
/// passing "ownership" of the `StateCell` between these contexts; moving a timer
/// from the `TimerEntry` to the driver requires _both_ holding the `&mut
/// TimerEntry` and the driver lock, while moving it back (firing the timer)
/// requires only the driver lock.
pub(super) struct StateCell {
    /// Holds either the scheduled expiration time for this timer, or (if the
    /// timer has been fired and is unregistered), `u64::MAX`.
    state: AtomicU64,
    /// If the timer is fired (an Acquire order read on state shows
    /// `u64::MAX`), holds the result that should be returned from
    /// polling the timer. Otherwise, the contents are unspecified and reading
    /// without holding the driver lock is undefined behavior.
    result: UnsafeCell<TimerResult>,
    /// The currently-registered waker
    waker: AtomicWaker,
}

impl Default for StateCell {
    fn default() -> Self {
        Self::new()
    }
}

impl std::fmt::Debug for StateCell {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "StateCell({:?})", self.read_state())
    }
}

impl StateCell {
    fn new() -> Self {
        Self {
            state: AtomicU64::new(STATE_DEREGISTERED),
            result: UnsafeCell::new(Ok(())),
            waker: AtomicWaker::new(),
        }
    }

    fn is_pending(&self) -> bool {
        self.state.load(Ordering::Relaxed) == STATE_PENDING_FIRE
    }

    /// Returns the current expiration time, or None if not currently scheduled.
    fn when(&self) -> Option<u64> {
        let cur_state = self.state.load(Ordering::Relaxed);

        if cur_state == STATE_DEREGISTERED {
            None
        } else {
            Some(cur_state)
        }
    }

    /// If the timer is completed, returns the result of the timer. Otherwise,
    /// returns None and registers the waker.
    fn poll(&self, waker: &Waker) -> Poll<TimerResult> {
        // We must register first. This ensures that either `fire` will
        // observe the new waker, or we will observe a racing fire to have set
        // the state, or both.
        self.waker.register_by_ref(waker);

        self.read_state()
    }

    fn read_state(&self) -> Poll<TimerResult> {
        let cur_state = self.state.load(Ordering::Acquire);

        if cur_state == STATE_DEREGISTERED {
            // SAFETY: The driver has fired this timer; this involves writing
            // the result, and then writing (with release ordering) the state
            // field.
            Poll::Ready(unsafe { self.result.with(|p| *p) })
        } else {
            Poll::Pending
        }
    }

    /// Marks this timer as being moved to the pending list, if its scheduled
    /// time is not after `not_after`.
    ///
    /// If the timer is scheduled for a time after `not_after`, returns an Err
    /// containing the current scheduled time.
    ///
    /// SAFETY: Must hold the driver lock.
    unsafe fn mark_pending(&self, not_after: u64) -> Result<(), u64> {
        // Quick initial debug check to see if the timer is already fired. Since
        // firing the timer can only happen with the driver lock held, we know
        // we shouldn't be able to "miss" a transition to a fired state, even
        // with relaxed ordering.
        let mut cur_state = self.state.load(Ordering::Relaxed);

        loop {
            // improve the error message for things like
            // https://github.com/tokio-rs/tokio/issues/3675
            assert!(
                cur_state < STATE_MIN_VALUE,
                "mark_pending called when the timer entry is in an invalid state"
            );

            if cur_state > not_after {
                break Err(cur_state);
            }

            match self.state.compare_exchange_weak(
                cur_state,
                STATE_PENDING_FIRE,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => break Ok(()),
                Err(actual_state) => cur_state = actual_state,
            }
        }
    }

    /// Fires the timer, setting the result to the provided result.
    ///
    /// Returns:
    /// * `Some(waker)` - if fired and a waker needs to be invoked once the
    ///   driver lock is released
    /// * `None` - if fired and a waker does not need to be invoked, or if
    ///   already fired
    ///
    /// SAFETY: The driver lock must be held.
    unsafe fn fire(&self, result: TimerResult) -> Option<Waker> {
        // Quick initial check to see if the timer is already fired. Since
        // firing the timer can only happen with the driver lock held, we know
        // we shouldn't be able to "miss" a transition to a fired state, even
        // with relaxed ordering.
        let cur_state = self.state.load(Ordering::Relaxed);
        if cur_state == STATE_DEREGISTERED {
            return None;
        }

        // SAFETY: We assume the driver lock is held and the timer is not
        // fired, so only the driver is accessing this field.
        //
        // We perform a release-ordered store to state below, to ensure this
        // write is visible before the state update is visible.
        unsafe { self.result.with_mut(|p| *p = result) };

        self.state.store(STATE_DEREGISTERED, Ordering::Release);

        self.waker.take_waker()
    }

    /// Marks the timer as registered (poll will return None) and sets the
    /// expiration time.
    ///
    /// While this function is memory-safe, it should only be called from a
    /// context holding both `&mut TimerEntry` and the driver lock.
    fn set_expiration(&self, timestamp: u64) {
        debug_assert!(timestamp < STATE_MIN_VALUE);

        // We can use relaxed ordering because we hold the driver lock and will
        // fence when we release the lock.
        self.state.store(timestamp, Ordering::Relaxed);
    }

    /// Attempts to adjust the timer to a new timestamp.
    ///
    /// If the timer has already been fired, is pending firing, or the new
    /// timestamp is earlier than the old timestamp, (or occasionally
    /// spuriously) returns Err without changing the timer's state. In this
    /// case, the timer must be deregistered and re-registered.
    fn extend_expiration(&self, new_timestamp: u64) -> Result<(), ()> {
        let mut prior = self.state.load(Ordering::Relaxed);
        loop {
            if new_timestamp < prior || prior >= STATE_MIN_VALUE {
                return Err(());
            }

            match self.state.compare_exchange_weak(
                prior,
                new_timestamp,
                Ordering::AcqRel,
                Ordering::Acquire,
            ) {
                Ok(_) => return Ok(()),
                Err(true_prior) => prior = true_prior,
            }
        }
    }

    /// Returns true if the state of this timer indicates that the timer might
    /// be registered with the driver. This check is performed with relaxed
    /// ordering, but is conservative - if it returns false, the timer is
    /// definitely _not_ registered.
    pub(super) fn might_be_registered(&self) -> bool {
        self.state.load(Ordering::Relaxed) != STATE_DEREGISTERED
    }
}

pin_project! {
    // A timer entry.
    //
    // This is the handle to a timer that is controlled by the requester of the
    // timer. As this participates in intrusive data structures, it must be pinned
    // before polling.
    #[derive(Debug)]
    pub(crate) struct TimerEntry {
        // Arc reference to the runtime handle. We can only free the driver after
        // deregistering everything from their respective timer wheels.
        driver: scheduler::Handle,
        // Shared inner structure; this is part of an intrusive linked list, and
        // therefore other references can exist to it while mutable references to
        // Entry exist.
        //
        // This is manipulated only under the inner mutex.
        #[pin]
        inner: TimerShared,
    }

    impl PinnedDrop for TimerEntry {
        fn drop(this: Pin<&mut Self>) {
            this.cancel();
        }
    }
}

unsafe impl Send for TimerEntry {}
unsafe impl Sync for TimerEntry {}

/// An `TimerHandle` is the (non-enforced) "unique" pointer from the driver to the
/// timer entry. Generally, at most one `TimerHandle` exists for a timer at a time
/// (enforced by the timer state machine).
///
/// SAFETY: An `TimerHandle` is essentially a raw pointer, and the usual caveats
/// of pointer safety apply. In particular, `TimerHandle` does not itself enforce
/// that the timer does still exist; however, normally an `TimerHandle` is created
/// immediately before registering the timer, and is consumed when firing the
/// timer, to help minimize mistakes. Still, because `TimerHandle` cannot enforce
/// memory safety, all operations are unsafe.
#[derive(Debug)]
pub(crate) struct TimerHandle {
    inner: NonNull<TimerShared>,
}

/// The shared state structure of a timer. This structure is shared between the
/// frontend (`Entry`) and driver backend.
///
/// Note that this structure is located inside the `TimerEntry` structure.
pub(crate) struct TimerShared {
    /// A link within the doubly-linked list of timers on a particular level and
    /// slot. Valid only if state is equal to Registered.
    ///
    /// Only accessed under the entry lock.
    pointers: linked_list::Pointers<TimerShared>,

    /// The time when the [`TimerEntry`] was registered into the Wheel,
    /// [`STATE_DEREGISTERED`] means it is not registered.
    ///
    /// Generally owned by the driver, but is accessed by the entry when not
    /// registered.
    ///
    /// We use relaxed ordering for both loading and storing since this value
    /// is only accessed either when holding the driver lock or through mutable
    /// references to [`TimerEntry`].
    registered_when: AtomicU64,

    /// Current state. This records whether the timer entry is currently under
    /// the ownership of the driver, and if not, its current state (not
    /// complete, fired, error, etc).
    state: StateCell,

    _p: PhantomPinned,
}

unsafe impl Send for TimerShared {}
unsafe impl Sync for TimerShared {}

impl std::fmt::Debug for TimerShared {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("TimerShared")
            .field(
                "registered_when",
                &self.registered_when.load(Ordering::Relaxed),
            )
            .field("state", &self.state)
            .finish()
    }
}

generate_addr_of_methods! {
    impl<> TimerShared {
        unsafe fn addr_of_pointers(self: NonNull<Self>) -> NonNull<linked_list::Pointers<TimerShared>> {
            &self.pointers
        }
    }
}

impl TimerShared {
    pub(super) fn new() -> Self {
        Self {
            registered_when: AtomicU64::new(0),
            pointers: linked_list::Pointers::new(),
            state: StateCell::default(),
            _p: PhantomPinned,
        }
    }

    /// Gets the cached time-of-expiration value.
    pub(super) fn registered_when(&self) -> u64 {
        // Cached-when is only accessed under the driver lock, so we can use relaxed
        self.registered_when.load(Ordering::Relaxed)
    }

    /// Gets the true time-of-expiration value, and copies it into the cached
    /// time-of-expiration value.
    ///
    /// SAFETY: Must be called with the driver lock held, and when this entry is
    /// not in any timer wheel lists.
    pub(super) unsafe fn sync_when(&self) -> u64 {
        let true_when = self.true_when();

        self.registered_when.store(true_when, Ordering::Relaxed);

        true_when
    }

    /// Sets the cached time-of-expiration value.
    ///
    /// SAFETY: Must be called with the driver lock held, and when this entry is
    /// not in any timer wheel lists.
    unsafe fn set_registered_when(&self, when: u64) {
        self.registered_when.store(when, Ordering::Relaxed);
    }

    /// Returns the true time-of-expiration value, with relaxed memory ordering.
    pub(super) fn true_when(&self) -> u64 {
        self.state.when().expect("Timer already fired")
    }

    /// Sets the true time-of-expiration value, even if it is less than the
    /// current expiration or the timer is deregistered.
    ///
    /// SAFETY: Must only be called with the driver lock held and the entry not
    /// in the timer wheel.
    pub(super) unsafe fn set_expiration(&self, t: u64) {
        self.state.set_expiration(t);
        self.registered_when.store(t, Ordering::Relaxed);
    }

    /// Sets the true time-of-expiration only if it is after the current.
    pub(super) fn extend_expiration(&self, t: u64) -> Result<(), ()> {
        self.state.extend_expiration(t)
    }

    /// Returns a `TimerHandle` for this timer.
    pub(super) fn handle(&self) -> TimerHandle {
        TimerHandle {
            inner: NonNull::from(self),
        }
    }

    /// Returns true if the state of this timer indicates that the timer might
    /// be registered with the driver. This check is performed with relaxed
    /// ordering, but is conservative - if it returns false, the timer is
    /// definitely _not_ registered.
    pub(super) fn might_be_registered(&self) -> bool {
        self.state.might_be_registered()
    }
}

unsafe impl linked_list::Link for TimerShared {
    type Handle = TimerHandle;

    type Target = TimerShared;

    fn as_raw(handle: &Self::Handle) -> NonNull<Self::Target> {
        handle.inner
    }

    unsafe fn from_raw(ptr: NonNull<Self::Target>) -> Self::Handle {
        TimerHandle { inner: ptr }
    }

    unsafe fn pointers(
        target: NonNull<Self::Target>,
    ) -> NonNull<linked_list::Pointers<Self::Target>> {
        unsafe { TimerShared::addr_of_pointers(target) }
    }
}

// ===== impl Entry =====

impl TimerEntry {
    pub(crate) fn new(handle: scheduler::Handle) -> Self {
        Self {
            driver: handle,
            inner: TimerShared::new(),
        }
    }

    pub(crate) fn init(self: Pin<&mut Self>, deadline: u64) {
        unsafe {
            self.driver()
                .reregister(&self.driver.driver().io, deadline, (&self.inner).into());
        }
    }

    pub(crate) fn is_elapsed(&self) -> bool {
        // Is this timer still in the timer wheel?
        !self.inner.might_be_registered()
    }

    /// Cancels and deregisters the timer. This operation is irreversible.
    pub(crate) fn cancel(self: Pin<&mut Self>) {
        // We need to perform an acq/rel fence with the driver thread, and the
        // simplest way to do so is to grab the driver lock.
        //
        // Why is this necessary? We're about to release this timer's memory for
        // some other non-timer use. However, we've been doing a bunch of
        // relaxed (or even non-atomic) writes from the driver thread, and we'll
        // be doing more from _this thread_ (as this memory is interpreted as
        // something else).
        //
        // It is critical to ensure that, from the point of view of the driver,
        // those future non-timer writes happen-after the timer is fully fired,
        // and from the purpose of this thread, the driver's writes all
        // happen-before we drop the timer. This in turn requires us to perform
        // an acquire-release barrier in _both_ directions between the driver
        // and dropping thread.
        //
        // The lock acquisition in clear_entry serves this purpose. All of the
        // driver manipulations happen with the lock held, so we can just take
        // the lock and be sure that this drop happens-after everything the
        // driver did so far and happens-before everything the driver does in
        // the future. While we have the lock held, we also go ahead and
        // deregister the entry if necessary.
        unsafe { self.driver().clear_entry(NonNull::from(&self.inner)) };
    }

    pub(crate) fn reset(self: Pin<&mut Self>, deadline: u64) {
        if self.inner.extend_expiration(deadline).is_ok() {
            return;
        }

        unsafe {
            self.driver()
                .reregister(&self.driver.driver().io, deadline, (&self.inner).into());
        }
    }

    pub(crate) fn poll_elapsed(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), super::Error>> {
        assert!(
            !self.driver().is_shutdown(),
            "{}",
            crate::util::error::RUNTIME_SHUTTING_DOWN_ERROR
        );

        self.inner.state.poll(cx.waker())
    }

    fn driver(&self) -> &super::Handle {
        self.driver.driver().time()
    }
}

impl TimerHandle {
    pub(super) unsafe fn registered_when(&self) -> u64 {
        unsafe { self.inner.as_ref().registered_when() }
    }

    pub(super) unsafe fn sync_when(&self) -> u64 {
        unsafe { self.inner.as_ref().sync_when() }
    }

    pub(super) unsafe fn is_pending(&self) -> bool {
        unsafe { self.inner.as_ref().state.is_pending() }
    }

    /// Forcibly sets the true and cached expiration times to the given tick.
    ///
    /// SAFETY: The caller must ensure that the handle remains valid, the driver
    /// lock is held, and that the timer is not in any wheel linked lists.
    pub(super) unsafe fn set_expiration(&self, tick: u64) {
        unsafe {
            self.inner.as_ref().set_expiration(tick);
        }
    }

    /// Attempts to mark this entry as pending. If the expiration time is after
    /// `not_after`, however, returns an Err with the current expiration time.
    ///
    /// If an `Err` is returned, the `registered_when` value will be updated to this
    /// new expiration time.
    ///
    /// SAFETY: The caller must ensure that the handle remains valid, the driver
    /// lock is held, and that the timer is not in any wheel linked lists.
    /// After returning Ok, the entry must be added to the pending list.
    pub(super) unsafe fn mark_pending(&self, not_after: u64) -> Result<(), u64> {
        match unsafe { self.inner.as_ref().state.mark_pending(not_after) } {
            Ok(()) => {
                // mark this as being on the pending queue in registered_when
                unsafe {
                    self.inner.as_ref().set_registered_when(STATE_DEREGISTERED);
                }
                Ok(())
            }
            Err(tick) => {
                unsafe {
                    self.inner.as_ref().set_registered_when(tick);
                }
                Err(tick)
            }
        }
    }

    /// Attempts to transition to a terminal state. If the state is already a
    /// terminal state, does nothing.
    ///
    /// Because the entry might be dropped after the state is moved to a
    /// terminal state, this function consumes the handle to ensure we don't
    /// access the entry afterwards.
    ///
    /// Returns the last-registered waker, if any.
    ///
    /// SAFETY: The driver lock must be held while invoking this function, and
    /// the entry must not be in any wheel linked lists.
    pub(super) unsafe fn fire(self, completed_state: TimerResult) -> Option<Waker> {
        unsafe { self.inner.as_ref().state.fire(completed_state) }
    }
}