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task: elaborate safety comments in task deallocation (#5172)

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Alice Ryhl 2022-11-06 22:13:25 +01:00 committed by GitHub
parent 9884fe3394
commit 909439c9f5
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1 changed files with 29 additions and 12 deletions
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41
tokio/src/runtime/task/harness.rs
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@ -1,6 +1,6 @@
use crate::future::Future;
use crate::runtime::task::core::{Cell, Core, CoreStage, Header, Trailer};
use crate::runtime::task::state::Snapshot;
use crate::runtime::task::state::{Snapshot, State};
use crate::runtime::task::waker::waker_ref;
use crate::runtime::task::{JoinError, Notified, Schedule, Task};
@ -31,7 +31,11 @@ where
}
fn header(&self) -> &Header {
unsafe { &self.cell.as_ref().header }
unsafe { &*self.header_ptr().as_ptr() }
}
fn state(&self) -> &State {
&self.header().state
}
fn trailer(&self) -> &Trailer {
@ -95,7 +99,7 @@ where
fn poll_inner(&self) -> PollFuture {
use super::state::{TransitionToIdle, TransitionToRunning};
match self.header().state.transition_to_running() {
match self.state().transition_to_running() {
TransitionToRunning::Success => {
let header_ptr = self.header_ptr();
let waker_ref = waker_ref::<T, S>(&header_ptr);
@ -113,7 +117,7 @@ where
return PollFuture::Complete;
}
match self.header().state.transition_to_idle() {
match self.state().transition_to_idle() {
TransitionToIdle::Ok => PollFuture::Done,
TransitionToIdle::OkNotified => PollFuture::Notified,
TransitionToIdle::OkDealloc => PollFuture::Dealloc,
@ -143,7 +147,7 @@ where
/// there is nothing further to do. When the task completes running, it will
/// notice the `CANCELLED` bit and finalize the task.
pub(super) fn shutdown(self) {
if !self.header().state.transition_to_shutdown() {
if !self.state().transition_to_shutdown() {
// The task is concurrently running. No further work needed.
self.drop_reference();
return;
@ -163,6 +167,19 @@ where
// Check causality
self.core().stage.with_mut(drop);
// Safety: The caller of this method just transitioned our ref-count to
// zero, so it is our responsibility to release the allocation.
//
// We don't hold any references into the allocation at this point, but
// it is possible for another thread to still hold a `&State` into the
// allocation if that other thread has decremented its last ref-count,
// but has not yet returned from the relevant method on `State`.
//
// However, the `State` type consists of just an `AtomicUsize`, and an
// `AtomicUsize` wraps the entirety of its contents in an `UnsafeCell`.
// As explained in the documentation for `UnsafeCell`, such references
// are allowed to be dangling after their last use, even if the
// reference has not yet gone out of scope.
unsafe {
drop(Box::from_raw(self.cell.as_ptr()));
}
@ -187,7 +204,7 @@ where
pub(super) fn drop_join_handle_slow(self) {
// Try to unset `JOIN_INTEREST`. This must be done as a first step in
// case the task concurrently completed.
if self.header().state.unset_join_interested().is_err() {
if self.state().unset_join_interested().is_err() {
// It is our responsibility to drop the output. This is critical as
// the task output may not be `Send` and as such must remain with
// the scheduler or `JoinHandle`. i.e. if the output remains in the
@ -214,7 +231,7 @@ where
/// the shutdown. This is necessary to avoid the shutdown happening in the
/// wrong thread for non-Send tasks.
pub(super) fn remote_abort(self) {
if self.header().state.transition_to_notified_and_cancel() {
if self.state().transition_to_notified_and_cancel() {
// The transition has created a new ref-count, which we turn into
// a Notified and pass to the task.
//
@ -237,7 +254,7 @@ where
pub(super) fn wake_by_val(self) {
use super::state::TransitionToNotifiedByVal;
match self.header().state.transition_to_notified_by_val() {
match self.state().transition_to_notified_by_val() {
TransitionToNotifiedByVal::Submit => {
// The caller has given us a ref-count, and the transition has
// created a new ref-count, so we now hold two. We turn the new
@ -267,7 +284,7 @@ where
pub(super) fn wake_by_ref(&self) {
use super::state::TransitionToNotifiedByRef;
match self.header().state.transition_to_notified_by_ref() {
match self.state().transition_to_notified_by_ref() {
TransitionToNotifiedByRef::Submit => {
// The transition above incremented the ref-count for a new task
// and the caller also holds a ref-count. The caller's ref-count
@ -282,7 +299,7 @@ where
}
pub(super) fn drop_reference(self) {
if self.header().state.ref_dec() {
if self.state().ref_dec() {
self.dealloc();
}
}
@ -299,7 +316,7 @@ where
// The future has completed and its output has been written to the task
// stage. We transition from running to complete.
let snapshot = self.header().state.transition_to_complete();
let snapshot = self.state().transition_to_complete();
// We catch panics here in case dropping the future or waking the
// JoinHandle panics.
@ -319,7 +336,7 @@ where
// The task has completed execution and will no longer be scheduled.
let num_release = self.release();
if self.header().state.transition_to_terminal(num_release) {
if self.state().transition_to_terminal(num_release) {
self.dealloc();
}
}