Re: [PATCH] Unlock crtc mutex acquired in drm_modeset_lock_crtc()

Re: [PATCH] Unlock crtc mutex acquired in drm_modeset_lock_crtc()

[Daniel Vetter]

On Mon, Sep 8, 2014 at 7:27 AM, Michael N. Henry <mnhenry333@gmail.com> wrote:
> crtc->mutex is acquired in drm_modeset_lock_crtc(). The next call
> to drm_modeset_unlock_crt() would not unlock it, causing a deadlock
> during the next call to drm_modest_lock_crtc().
> ---
>  drivers/gpu/drm/drm_modeset_lock.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
> index 474e4d1..fab3aa8 100644
> --- a/drivers/gpu/drm/drm_modeset_lock.c
> +++ b/drivers/gpu/drm/drm_modeset_lock.c
> @@ -236,6 +236,8 @@ void drm_modeset_unlock_crtc(struct drm_crtc *crtc)
>         drm_modeset_drop_locks(ctx);
>         drm_modeset_acquire_fini(ctx);
>
> +       drm_modeset_unlock(&crtc->mutex);
> +

drm_mode_drop_locks should drop all mutexes (and so also the crtc
mutex acquired with ctx. Also acquire_fini should complain if you
unlock afterwards.

I suspect that some other caller completely forgets to unlock the crtc
somehow, but I can't see where that happens. Can you please re-run
with CONFIG_PROVE_LOCKING enabled in your kernel build?

And a small nitpick on your patch submission: When you submit just one
patch, please don't do a cover letter - all the information you
provide in there (backtrace, bisect result) should be part of the
commit message anyway. And your patch is missing the signed-off-by
line, so can't be merged. And please also don't forget to cc relevant
mailing lists.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch