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Subject: Re: [PATCH RFC] io_uring: make signalfd work with io_uring (and aio)
 POLL
To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>
References: <58059c9c-adf9-1683-99f5-7e45280aea87@kernel.dk>
 <58246851-fa45-a72d-2c42-7e56461ec04e@kernel.dk>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <ec3526fb-948a-70c0-4a7b-866d6cd6a788@rasmusvillemoes.dk>
Date:   Thu, 14 Nov 2019 10:19:58 +0100
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On 14/11/2019 05.49, Jens Axboe wrote:
> On 11/13/19 9:31 PM, Jens Axboe wrote:
>> This is a case of "I don't really know what I'm doing, but this works
>> for me". Caveat emptor, but I'd love some input on this.
>>
>> I got a bug report that using the poll command with signalfd doesn't
>> work for io_uring. The reporter also noted that it doesn't work with the
>> aio poll implementation either. So I took a look at it.
>>
>> What happens is that the original task issues the poll request, we call
>> ->poll() (which ends up with signalfd for this fd), and find that
>> nothing is pending. Then we wait, and the poll is passed to async
>> context. When the requested signal comes in, that worker is woken up,
>> and proceeds to call ->poll() again, and signalfd unsurprisingly finds
>> no signals pending, since it's the async worker calling it.
>>
>> That's obviously no good. The below allows you to pass in the task in
>> the poll_table, and it does the right thing for me, signal is delivered
>> and the correct mask is checked in signalfd_poll().
>>
>> Similar patch for aio would be trivial, of course.
> 
> From the probably-less-nasty category, Jann Horn helpfully pointed out
> that it'd be easier if signalfd just looked at the task that originally
> created the fd instead. That looks like the below, and works equally
> well for the test case at hand.

Eh, how should that work? If I create a signalfd() and fork(), the
child's signalfd should only be concerned with signals sent to the
child. Not to mention what happens after the parent dies and the child
polls its fd.

Or am I completely confused?

Rasmus