[PATCH] drm: Fix deadlock retry loop in page_flip_ioctl

[Tommi Rantala]

2017-05-22 16:59 GMT+03:00 Daniel Vetter <daniel.vetter at ffwll.ch>:
> I failed to properly onion-wrap the unwind code: We acquire the vblank
> reference before we start with the wait-wound locking dance, hence we
> must make sure we retry before we drop the reference. Oops.
>
> v2: The vblank_put must be after the frambuffer_put (Michel). I suck at
> unwrapping code that doesn't use separate labels for each stage, but
> checks each pointer first ... While re-reading everything I also
> realized that we must clean up the fb refcounts, and specifically
> plane->old_fb before we drop the locks, either in the final unlocking,
> or in the w/w retry path. Hence the correct fix is to drop the
> vblank_put to the very bottom.
>
> Fixes: 29dc0d1de182 ("drm: Roll out acquire context for the page_flip ioctl")
> Cc: Harry Wentland <harry.wentland at amd.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Sean Paul <seanpaul at chromium.org>
> Cc: David Airlie <airlied at linux.ie>
> Cc: dri-devel at lists.freedesktop.org
> Reported-by: Tommi Rantala <tt.rantala at gmail.com>
> Cc: Tommi Rantala <tt.rantala at gmail.com>
> Cc: Michel Dänzer <michel at daenzer.net>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

Thanks, confirmed that this fixes the warning that I was seeing!

Tested-by: Tommi Rantala <tt.rantala at gmail.com>

> ---
>  drivers/gpu/drm/drm_plane.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index fedd4d60d9cd..5dc8c4350602 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -948,8 +948,6 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
>         }
>
>  out:
> -       if (ret && crtc->funcs->page_flip_target)
> -               drm_crtc_vblank_put(crtc);
>         if (fb)
>                 drm_framebuffer_put(fb);
>         if (crtc->primary->old_fb)
> @@ -964,5 +962,8 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
>         drm_modeset_drop_locks(&ctx);
>         drm_modeset_acquire_fini(&ctx);
>
> +       if (ret && crtc->funcs->page_flip_target)
> +               drm_crtc_vblank_put(crtc);
> +
>         return ret;
>  }
> --
> 2.11.0
>