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From:   Alexandre Courbot <gnurou@gmail.com>
Date:   Wed, 26 Aug 2020 20:21:19 +0900
Message-ID: <CAAVeFuLnfeBG89hVsM8JuFaCP+s7RTZe_OvFM6WPVH0dnkAZNA@mail.gmail.com>
Subject: Re: [PATCH 1/2] media: v4l2-mem2mem: consider OUTPUT queue first when polling
To:     Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Nicolas Dufresne <nicolas.dufresne@collabora.com>,
        linux-media <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Aug 26, 2020 at 1:08 PM Ezequiel Garcia
<ezequiel@vanguardiasur.com.ar> wrote:
>
> Hi Alexandre,
>
> On Tue, 25 Aug 2020 at 11:56, Alexandre Courbot <gnurou@gmail.com> wrote:
> >
> > If poll() is called on a m2m device with the EPOLLOUT event after the
> > last buffer of the CAPTURE queue is dequeued, any buffer available on
> > OUTPUT queue will never be signaled because v4l2_m2m_poll_for_data()
> > starts by checking whether dst_q->last_buffer_dequeued is set and
> > returns EPOLLIN in this case, without looking at the state of the OUTPUT
> > queue.
> >
> > Fix this by checking the state of the OUTPUT queue before considering
> > that early-return case.
> >
> > This also has the side-effect of bringing the two blocks of code dealing
> > with the CAPTURE queue next to one another, and saves us one spin
> > lock/unlock cycle, for what it's worth.
> >
> > Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
>
> Change looks good to me.
>
> Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
>
> Do you think it qualifies for -stable? The issue has been
> here since the dawn of time.

Indeed, and this should be quite a rare corner case. I will leave that
call to the maintainers.

>
> Thanks,
> Ezequiel
>
> > ---
> >  drivers/media/v4l2-core/v4l2-mem2mem.c | 23 +++++++++++------------
> >  1 file changed, 11 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> > index 95a8f2dc5341d..0d0192119af20 100644
> > --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> > +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> > @@ -862,6 +862,15 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
> >              list_empty(&dst_q->queued_list)))
> >                 return EPOLLERR;
> >
> > +       spin_lock_irqsave(&src_q->done_lock, flags);
> > +       if (!list_empty(&src_q->done_list))
> > +               src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
> > +                                               done_entry);
> > +       if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
> > +                       || src_vb->state == VB2_BUF_STATE_ERROR))
> > +               rc |= EPOLLOUT | EPOLLWRNORM;
> > +       spin_unlock_irqrestore(&src_q->done_lock, flags);
> > +
> >         spin_lock_irqsave(&dst_q->done_lock, flags);
> >         if (list_empty(&dst_q->done_list)) {
> >                 /*
> > @@ -870,21 +879,11 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
> >                  */
> >                 if (dst_q->last_buffer_dequeued) {
> >                         spin_unlock_irqrestore(&dst_q->done_lock, flags);
> > -                       return EPOLLIN | EPOLLRDNORM;
> > +                       rc |= EPOLLIN | EPOLLRDNORM;
> > +                       return rc;
> >                 }
> >         }
> > -       spin_unlock_irqrestore(&dst_q->done_lock, flags);
> >
> > -       spin_lock_irqsave(&src_q->done_lock, flags);
> > -       if (!list_empty(&src_q->done_list))
> > -               src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
> > -                                               done_entry);
> > -       if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
> > -                       || src_vb->state == VB2_BUF_STATE_ERROR))
> > -               rc |= EPOLLOUT | EPOLLWRNORM;
> > -       spin_unlock_irqrestore(&src_q->done_lock, flags);
> > -
> > -       spin_lock_irqsave(&dst_q->done_lock, flags);
> >         if (!list_empty(&dst_q->done_list))
> >                 dst_vb = list_first_entry(&dst_q->done_list, struct vb2_buffer,
> >                                                 done_entry);
> > --
> > 2.28.0
> >