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

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Date:   Wed, 24 Jun 2020 19:56:58 +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 22:01, Jens Axboe wrote:
> On 6/23/20 5:57 AM, Pavel Begunkov wrote:
>> 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()).
> 
> Yes it can. The primary example would be a polled bio that gets split, into
> let's say 4 bio's. First one queues fine, but one of the subsequent ones
> run into request allocation failures and it gets marked as -EAGAIN.

Right, thanks for the explanation. And that's the case where io_uring fails.
Now I tested all kinds of -EAGAIN to be sure.

-- 
Pavel Begunkov