Messages in this thread Patch in this message | | | Date | Thu, 21 Dec 2023 08:54:17 -0800 | From | Boqun Feng <> | Subject | Re: [PATCH v2 3/4] rust: sync: add `CondVar::wait_timeout` |
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On Wed, Dec 20, 2023 at 11:31:05AM +0000, Benno Lossin wrote: > On 12/16/23 16:31, Alice Ryhl wrote: > > @@ -102,7 +105,12 @@ pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self > > }) > > } > > > > - fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guard<'_, T, B>) { > > + fn wait_internal<T: ?Sized, B: Backend>( > > + &self, > > + wait_state: u32, > > + guard: &mut Guard<'_, T, B>, > > + timeout: c_long, > > + ) -> c_long { > > let wait = Opaque::<bindings::wait_queue_entry>::uninit(); > > > > // SAFETY: `wait` points to valid memory. > > @@ -113,11 +121,13 @@ fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guar > > bindings::prepare_to_wait_exclusive(self.wait_list.get(), wait.get(), wait_state as _) > > }; > > > > - // SAFETY: No arguments, switches to another thread. > > - guard.do_unlocked(|| unsafe { bindings::schedule() }); > > + // SAFETY: Switches to another thread. The timeout can be any number. > > + let ret = guard.do_unlocked(|| unsafe { bindings::schedule_timeout(timeout) }); > > I am not sure what exactly the safety requirements of `schedule_timeout` > are. I looked at the function and saw that the timout should not be > negative. But aside from that only the the context switching should be > relevant. What things are not allowed to do when calling `schedule` > (aside from the stuff that klint catches)?
One thing is that you probably don't want to call `schedule` with task state being TASK_DEAD, if so the `schedule` would be counted as `ARef<Task>::drop()`, see __schedule() -> context_switch() -> finish_context_switch(), and the task may be freed after that, which free the stack of the task, and anything that references a object on the stack would be a UAF. On the other hand, if the task state is not TASK_DEAD, `schedule*()` should be a no-op regarding memory safety.
> Because if there are none, then I would put the "switches to another > thread" part into a normal comment. >
I think it's possible to make schedule_timeout() a safe function: we can define setting task state TASK_DEAD as an unsafe operation, whose safety requirement is something like: "Must ensure that if some code can reference a memory object that belongs to the task (e.g. a stack variable) after the task calls a followed `schedule()`, the code must also hold an additional reference count to the task."
Yes, it might be out of the scope of this patchset though.
Regards, Boqun
> -- > Cheers, > Benno > > > > > // SAFETY: Both `wait` and `wait_list` point to valid memory. > > unsafe { bindings::finish_wait(self.wait_list.get(), wait.get()) }; > > + > > + ret > > } >
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