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
