linux-kernel - Re: [PATCH 1/4] io_uring: fix hanging iopoll in case of -EAGAIN

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Date:   Tue, 23 Jun 2020 14:57:38 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] io_uring: fix hanging iopoll in case of -EAGAIN

On 23/06/2020 05:18, Jens Axboe wrote:
> On 6/22/20 8:07 PM, Jens Axboe wrote:
>> On 6/22/20 4:16 PM, Pavel Begunkov wrote:
>>> io_do_iopoll() won't do anything with a request unless
>>> req->iopoll_completed is set. So io_complete_rw_iopoll() has to set
>>> it, otherwise io_do_iopoll() will poll a file again and again even
>>> though the request of interest was completed long ago.
>>
>> I need to look at this again, because with this change, I previously
>> got various use-after-free. I haven't seen any issues with it, but
>> I agree, from a quick look that I'm not quite sure how it's currently
>> not causing hangs. Yet I haven't seen any, with targeted -EAGAIN
>> testing.

Can io_complete_rw_iopoll() get -EAGAIN after being successfully enqueued
(i.e. EIOCBQUEUED)? It's reliably fails for me, because my hacked nullblk
_can_ (i.e. probabilistically returns BLK_STS_AGAIN from ->iopoll()).

> 
> Ah I think I know what it is - if we run into:
> 
> if (req->result == -EAGAIN)
> 	return -EAGAIN
> 
> in io_issue_sqe() and race with it, we'll reissue twice potentially.
> So the above isn't quite enough, we'll need something a bit broader.

I see, I'll deal with it.


-- 
Pavel Begunkov