drm: Wake up next in drm_read() chain if we are forced to putback the event

[ Upstream commit 60b801999c48b6c1dd04e653a38e2e613664264e ]

After an event is sent, we try to copy it into the user buffer of the
first waiter in drm_read() and if the user buffer doesn't have enough
room we put it back onto the list. However, we didn't wake up any
subsequent waiter, so that event may sit on the list until either a new
vblank event is sent or a new waiter appears. Rare, but in the worst
case may lead to a stuck process.

Testcase: igt/drm_read/short-buffer-wakeup
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170804082328.17173-1-chris@chris-wilson.co.uk
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 03244b3c985d7abe6d2c2743bf52b701fa01a7db..3cf07f5063ff64f23fd43cfaf307d8064ee8784d 100644 (file)
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -525,6 +525,7 @@ ssize_t drm_read(struct file *filp, char __user *buffer,
                                file_priv->event_space -= length;
                                list_add(&e->link, &file_priv->event_list);
                                spin_unlock_irq(&dev->event_lock);
+                               wake_up_interruptible(&file_priv->event_wait);
                                break;
                        }