[Intel-gfx] [PATCH] drm: Fix deadlock retry loop in page_flip_ioctl

[Daniel Vetter]

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>
---
 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