Re: Canceled read requests never completed

[Jens Axboe <axboe@kernel.dk>]

On 1/18/22 4:36 PM, Jens Axboe wrote:
> On 1/18/22 1:05 PM, Florian Fischer wrote:
>>>> 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.
> 
> I think there are a couple of different cases here. Can you try the
> below patch? It's against current -git.

Cleaned it up and split it into functional bits, end result is here:

https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-5.17

-- 
Jens Axboe