Re: [PATCH 5/6] rust: hrtimer: document deadlock when starting a timer in its handler

[Andreas Hindborg]

"Gary Guo" <gary@xxxxxxxxxxx> writes:

> On Tue Aug 25, 2026 at 1:16 PM BST, Andreas Hindborg wrote:
>> The state machine documentation states that for pointer types that
>> implement `Clone`, the `start` operation may be issued while the timer
>> is in the **running** state, and that it is then equivalent to the
>> `restart` operation. That only holds when the operation is issued from
>> outside the timer handler.
>>
>> The `start` operation returns a `HrTimerHandle`, and dropping the
>> handle cancels the timer with `hrtimer_cancel()`, which blocks until a
>> running handler has returned. When `start` is issued from within the
>> handler, the handle is also dropped within the handler, so the cancel
>> waits for the very handler that issues it, and the handler deadlocks.
>
> That's not always true, you can start timer and store its handle elsewhere.
>
> I think the proper wording is that "cancelling a timer from within the timer
> handler will lead to deadlock".

Right, I'll update the wording and comment.


Best regards,
Andreas Hindborg