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Subject: Re: [PATCH 1/4] io_uring: fix hanging iopoll in case of -EAGAIN
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <d33e9006-b7ef-4925-ff3f-332ab655f2ae@kernel.dk>
Date:   Tue, 23 Jun 2020 13:01:29 -0600
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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.

>> 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.

Thanks!

-- 
Jens Axboe