Re: Canceled read requests never completed

[Florian Fischer <florian.fl.fischer@fau.de>]

> > After reading the io_uring_enter(2) man page a IORING_OP_ASYNC_CANCEL's return value of -EALREADY apparently
> > may not cause the request to terminate. At least that is our interpretation of "…res field will contain -EALREADY.
> > In this case, the request may or may not terminate."
> 
> I took a look at this, and my theory is that the request cancelation
> ends up happening right in between when the work item is moved between
> the work list and to the worker itself. The way the async queue works,
> the work item is sitting in a list until it gets assigned by a worker.
> When that assignment happens, it's removed from the general work list
> and then assigned to the worker itself. There's a small gap there where
> the work cannot be found in the general list, and isn't yet findable in
> the worker itself either.
> 
> Do you always see -ENOENT from the cancel when you get the hang
> condition?

No we also and actually more commonly observe cancel returning -EALREADY and the
canceled read request never gets completed.

As shown in the log snippet I included below.

> > The far more common situation with the reproducer and adding 1 to the eventfds in each loop
> > is that a request is not canceled and the cancel attempt returned with -EALREADY.
> > There is no progress because the writer has already finished its loop and the cancel
> > apparently does not really cancel the request.
> > 
> >   1 Starting iteration 996
> >   1 Prepared read request (evfd: 1, tag: 1)
> >   1 Submitted 1 requests -> 1 inflight
> >   1 Prepared read request (evfd: 2, tag: 2)
> >   1 Submitted 1 requests -> 2 inflight
> >   1 Prepared write request (evfd: 0)
> >   1 Submitted 1 requests -> 3 inflight
> >   1 Collect write completion: 8
> >   1 Prepared cancel request for read 1
> >   1 Prepared cancel request for read 2
> >   1 Submitted 2 requests -> 4 inflight
> >   1 Collect read 1 completion: -125 - Operation canceled
> >   1 Collect cancel read 1 completion: 0
> >   1 Collect cancel read 2 completion: -114 - Operation already in progress

^- the cancel returned with -EALREADY but the cancelled read (the second
prepared read request) is never completed.

> 
> I'll play with this a bit and see if we can't close this hole so the
> work is always reliably discoverable (and hence can get canceled).

Thanks for your effort!