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Make some linked list operations unsafe, and document their safety at usage

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This commit is contained in:
Emil Fresk 2023-02-18 09:43:06 +01:00 • committed by Henrik Tjäder
parent 002d0b0d16
commit 1cda61fbda
6 changed files with 66 additions and 40 deletions
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19
rtic-time/src/lib.rs
View file

@ -208,7 +208,8 @@ impl<Mono: Monotonic> TimerQueue<Mono> {
let mut link_ptr: Option<linked_list::Link<WaitingWaker<Mono>>> = None;
// Make this future `Drop`-safe, also shadow the original definition so we can't abuse it.
// Make this future `Drop`-safe
// SAFETY(link_ptr): Shadow the original definition of `link_ptr` so we can't abuse it.
let mut link_ptr =
LinkPtr(&mut link_ptr as *mut Option<linked_list::Link<WaitingWaker<Mono>>>);
let mut link_ptr2 = link_ptr.clone();
@ -226,18 +227,24 @@ impl<Mono: Monotonic> TimerQueue<Mono> {
}
// SAFETY: This pointer is only dereferenced here and on drop of the future
// which happens outside this `poll_fn`'s stack frame.
// which happens outside this `poll_fn`'s stack frame, so this mutable access cannot
// happen at the same time as `dropper` runs.
let link = unsafe { link_ptr2.get() };
if link.is_none() {
let mut link_ref = link.insert(Link::new(WaitingWaker {
let link_ref = link.insert(Link::new(WaitingWaker {
waker: cx.waker().clone(),
release_at: instant,
was_poped: AtomicBool::new(false),
}));
// SAFETY: The address to the link is stable as it is defined outside this stack
// frame.
let (was_empty, addr) = queue.insert(unsafe { Pin::new_unchecked(&mut link_ref) });
// SAFETY(new_unchecked): The address to the link is stable as it is defined
//outside this stack frame.
// SAFETY(insert): `link_ref` lifetime comes from `link_ptr` that is shadowed, and
// we make sure in `dropper` that the link is removed from the queue before
// dropping `link_ptr` AND `dropper` makes sure that the shadowed `link_ptr` lives
// until the end of the stack frame.
let (was_empty, addr) = unsafe { queue.insert(Pin::new_unchecked(&link_ref)) };
marker.store(addr, Ordering::Relaxed);
if was_empty {

18
rtic-time/src/linked_list.rs
View file

@ -93,10 +93,12 @@ impl<T: PartialOrd + Clone> LinkedList<T> {
/// Insert a new link into the linked list.
/// The return is (was_empty, address), where the address of the link is for use with `delete`.
pub fn insert(&self, val: Pin<&mut Link<T>>) -> (bool, usize) {
///
/// SAFETY: The pinned link must live until it is removed from this list.
pub unsafe fn insert(&self, val: Pin<&Link<T>>) -> (bool, usize) {
cs::with(|_| {
// SAFETY: This datastructure does not move the underlying value.
let val = unsafe { val.get_unchecked_mut() };
let val = val.get_ref();
let addr = val as *const _ as usize;
// Make sure all previous writes are visible
@ -111,7 +113,8 @@ impl<T: PartialOrd + Clone> LinkedList<T> {
let head_ref = if let Some(head_ref) = unsafe { head.as_ref() } {
head_ref
} else {
self.head.store(val, Ordering::Relaxed);
self.head
.store(val as *const _ as *mut _, Ordering::Relaxed);
return (true, addr);
};
@ -121,7 +124,8 @@ impl<T: PartialOrd + Clone> LinkedList<T> {
val.next.store(head, Ordering::Relaxed);
// `val` is now first in the queue
self.head.store(val, Ordering::Relaxed);
self.head
.store(val as *const _ as *mut _, Ordering::Relaxed);
return (false, addr);
}
@ -139,7 +143,8 @@ impl<T: PartialOrd + Clone> LinkedList<T> {
val.next.store(next, Ordering::Relaxed);
// Insert `val`
curr.next.store(val, Ordering::Relaxed);
curr.next
.store(val as *const _ as *mut _, Ordering::Relaxed);
return (false, addr);
}
@ -150,7 +155,8 @@ impl<T: PartialOrd + Clone> LinkedList<T> {
}
// No next, write link to last position in list
curr.next.store(val, Ordering::Relaxed);
curr.next
.store(val as *const _ as *mut _, Ordering::Relaxed);
(false, addr)
})