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Subject: Re: [PATCH 1/4] io_uring: fix hanging iopoll in case of -EAGAIN
From:   Jens Axboe <axboe@kernel.dk>
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Date:   Mon, 22 Jun 2020 20:18:49 -0600
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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.

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.

-- 
Jens Axboe