Re: io_uring_peek_cqe and EAGAIN

[Jens Axboe <axboe@kernel.dk>]

On 4/23/20 8:42 AM, William Dauchy wrote:
> Hello Jens,
> 
> Thank you for your answer on this newbie question :)
> 
> On Wed, Apr 22, 2020 at 10:57 PM Jens Axboe <axboe@kernel.dk> wrote:
>> I don't think the change is correct. That's not saying that the original
>> code is necessarily correct, though! Basically there are two cases there:
>>
>> 1) We haven't gotten a completion yet, we'll wait for it.
>> 2) We already found at least one completion. We don't want
>>    to _wait_ for more, but we can peek and see if there are more.
>>
>> Hence we don't want to turn case 2 into a loop, we should just
>> continue.
> 
> ok so in fact I think I understand that my usage is incorrect:
> 1- if I'm in the case of being able to do other things while waiting
> for data available using `io_uring_peek_cqe`, I should use it and come
> back later when getting a -EAGAIN.
> 2- it is useless to do a loop on `io_uring_peek_cqe` because in that
> case, I should simply do a `io_uring_wait_cqe`
> 
> is that correct?

Right, you rarely want to busy loop on io_uring_peek_cqe(), the normal
use case would be to use io_uring_wait_cqe() if you need to wait for a
completion to become available.

>> How is it currently failing for you?
> 
> While trying to open/read/close multiple files, I first thought that,
> because I had one successful `io_uring_wait_cqe`, I could then loop on
> `io_uring_peek_cqe` and get all my data. I now realise my assumption
> was completely wrong and this example was just written that way to
> show two different possibilities of getting results.

Ah ok, yes that sounds like a misunderstanding. Events are posted as
they become available, availability of one does not mean that everything
has completed.

-- 
Jens Axboe