linux-kernel - Re: [PATCH 0/2] media: v4l2-mem2mem: fix poll() bug

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Message-ID: <CAAEAJfBbxExQP7cAQnsAE1_Zh-onBKPt8is3S9qBEpx6MAywRg@mail.gmail.com>
Date:   Tue, 25 Aug 2020 19:10:47 -0300
From:   Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
To:     Alexandre Courbot <gnurou@...il.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        linux-media <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] media: v4l2-mem2mem: fix poll() bug

Hello Alex,

Thanks for the patch.

On Tue, Aug 25, 2020, 11:56 AM Alexandre Courbot <gnurou@...il.com> wrote:
>
> This addresses a very corner case that probably nobody ever encounters,
> but I have hit it when playing with vicoded so here is a tentative fix.
>

I'll try to make a more complete review soon, but meanwhile
I was thinking if it was possible to include a little kselftest program
for this issue, something CIs can pick-up and test corner cases like this,
making sure we don't regress on the issue.

(Or alternatively, v4l2-compliance?)

Thanks!
Ezequiel

> Patch 1/2 addresses the issue that when the last buffer of a m2m device
> has been dequeued, any attempt to poll with EPOLLOUT will result in only
> EPOLLIN being returned, even if OUTPUT buffers are still pending. The
> issue stems from the fact that the last buffer check if done first, and
> returns immediately if true.
>
> Patch 2/2 builds on the first one to (hopefully) clean up the code a bit
> and make the function flow easier to follow. Functionally speaking it is
> supposed to be a no-op and it can safely be dropped if the former code
> is preferred - the actual fix is in 1/2.
>
> Alexandre Courbot (2):
>   media: v4l2-mem2mem: consider OUTPUT queue first when polling
>   media: v4l2-mem2mem: simplify poll logic a bit
>
>  drivers/media/v4l2-core/v4l2-mem2mem.c | 42 +++++++++++---------------
>  1 file changed, 18 insertions(+), 24 deletions(-)
>
> --
> 2.28.0
>