Another comment. The subject line states v6.10, but this is a v6.9
regression. How about reverting the offending commit for v6.9, and
working on a better implementation for v6.11 ?
On Fri, May 10, 2024 at 03:02:25AM +0300, Laurent Pinchart wrote:
> Hi Hans,
>
> Thank you for the patch.
>
> On Wed, May 08, 2024 at 11:11:01AM +0200, Hans Verkuil wrote:
> > When logging the control values through VIDIOC_LOG_STATUS you could
> > get into a potential deadlock situation in the vivid driver:
> >
> > [Wed May 8 10:02:06 2024] Possible unsafe locking scenario:
> >
> > [Wed May 8 10:02:06 2024] CPU0 CPU1
> > [Wed May 8 10:02:06 2024] ---- ----
> > [Wed May 8 10:02:06 2024] lock(vivid_ctrls:1620:(hdl_vid_cap)->_lock);
> > [Wed May 8 10:02:06 2024] lock(vivid_ctrls:1608:(hdl_user_vid)->_lock);
> > [Wed May 8 10:02:06 2024] lock(vivid_ctrls:1620:(hdl_vid_cap)->_lock);
> > [Wed May 8 10:02:06 2024] lock(vivid_ctrls:1608:(hdl_user_vid)->_lock);
>
> This looks scary. How was this triggered, can you provide the full
> lockdep report ? I'm worried there could be other scenarios where the
> locks could be taken in different orders.
>
> > That's because both the main control handler was locked and the
> > inner control handler containing the control that you want to log.
> >
> > In this particular case there is no need to lock the inner control
> > handler since it is guaranteed that that handler won't delete controls
> > unexpectedly.
>
> All the v4l2_ctrl_type_ops operations currently rely on the control
> handler being locked when they're called. I don't like the idea of
> lifting that requirement without an audit of the implementations. We
> should then clearly document that the log operation can be called
> without the lock held.
>
> >
> > Fixes: 9801b5b28c692 ("media: v4l2-ctrls: show all owned controls in log_status")
> > Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
> > ---
> > drivers/media/v4l2-core/v4l2-ctrls-core.c | 4 ----
> > 1 file changed, 4 deletions(-)
> >
> > diff --git a/drivers/media/v4l2-core/v4l2-ctrls-core.c b/drivers/media/v4l2-core/v4l2-ctrls-core.c
> > index c59dd691f79f6..4e6748bd419cf 100644
> > --- a/drivers/media/v4l2-core/v4l2-ctrls-core.c
> > +++ b/drivers/media/v4l2-core/v4l2-ctrls-core.c
> > @@ -2518,11 +2518,7 @@ static void log_ctrl(const struct v4l2_ctrl_handler *hdl,
>
> You can also drop the hdl argument to this function, which was
> introduced by the commit referenced by Fixes:.
>
> >
> > pr_info("%s%s%s: ", prefix, colon, ctrl->name);
> >
> > - if (ctrl->handler != hdl)
> > - v4l2_ctrl_lock(ctrl);
> > ctrl->type_ops->log(ctrl);
> > - if (ctrl->handler != hdl)
> > - v4l2_ctrl_unlock(ctrl);
> >
> > if (ctrl->flags & (V4L2_CTRL_FLAG_INACTIVE |
> > V4L2_CTRL_FLAG_GRABBED |
--
Regards,
Laurent Pinchart
