[PATCH] drm/i915: don't set unpin_work if vblank_get fails

[Jesse Barnes <jbarnes@virtuousgeek.org>]

This fixes a race where we may try to finish a page flip and decrement
the refcount even if our vblank_get failed and we ended up with a
spurious flip pending interrupt.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=34211.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 2319f62..0910537 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -6896,6 +6896,10 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 	work->old_fb_obj = intel_fb->obj;
 	INIT_WORK(&work->work, intel_unpin_work_fn);
 
+	ret = drm_vblank_get(dev, intel_crtc->pipe);
+	if (ret)
+		goto free_work;
+
 	/* We borrow the event spin lock for protecting unpin_work */
 	spin_lock_irqsave(&dev->event_lock, flags);
 	if (intel_crtc->unpin_work) {
@@ -6906,6 +6910,11 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 		return -EBUSY;
 	}
 	intel_crtc->unpin_work = work;
+	/*
+	 * Past this point, if we fail we'll let the flip completion code
+	 * clean up the vblank refcount and pin work.  It'll be a spurious
+	 * completion, but we handle that case.
+	 */
 	spin_unlock_irqrestore(&dev->event_lock, flags);
 
 	intel_fb = to_intel_framebuffer(fb);
@@ -6919,10 +6928,6 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 
 	crtc->fb = fb;
 
-	ret = drm_vblank_get(dev, intel_crtc->pipe);
-	if (ret)
-		goto cleanup_objs;
-
 	work->pending_flip_obj = obj;
 
 	work->enable_stall_check = true;
@@ -6945,7 +6950,6 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 
 cleanup_pending:
 	atomic_sub(1 << intel_crtc->plane, &work->old_fb_obj->pending_flip);
-cleanup_objs:
 	drm_gem_object_unreference(&work->old_fb_obj->base);
 	drm_gem_object_unreference(&obj->base);
 	mutex_unlock(&dev->struct_mutex);
@@ -6953,7 +6957,7 @@ cleanup_objs:
 	spin_lock_irqsave(&dev->event_lock, flags);
 	intel_crtc->unpin_work = NULL;
 	spin_unlock_irqrestore(&dev->event_lock, flags);
-
+free_work:
 	kfree(work);
 
 	return ret;