drm: Serialise multiple event readers

The previous patch reintroduced a race condition whereby a failure in
one reader may allow a second reader to see out-of-order events.
Introduce a mutex to serialise readers so that an event is completed in
its entirety before another reader may process an event. The two readers
may race against each other, but the events each retrieves are in the
correct order.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1448462343-2072-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_fops.c

@@ -172,6 +172,8 @@ static int drm_open_helper(struct file *filp, struct drm_minor *minor)
 	init_waitqueue_head(&priv->event_wait);
 	priv->event_space = 4096; /* set aside 4k for event buffer */
 
+	mutex_init(&priv->event_read_lock);
+
 	if (drm_core_check_feature(dev, DRIVER_GEM))
 		drm_gem_open(dev, priv);
 
@@ -483,11 +485,15 @@ ssize_t drm_read(struct file *filp, char __user *buffer,
 {
 	struct drm_file *file_priv = filp->private_data;
 	struct drm_device *dev = file_priv->minor->dev;
-	ssize_t ret = 0;
+	ssize_t ret;
 
 	if (!access_ok(VERIFY_WRITE, buffer, count))
 		return -EFAULT;
 
+	ret = mutex_lock_interruptible(&file_priv->event_read_lock);
+	if (ret)
+		return ret;
+
 	for (;;) {
 		struct drm_pending_event *e = NULL;
 
@@ -509,12 +515,13 @@ ssize_t drm_read(struct file *filp, char __user *buffer,
 				break;
 			}
 
+			mutex_unlock(&file_priv->event_read_lock);
 			ret = wait_event_interruptible(file_priv->event_wait,
 						       !list_empty(&file_priv->event_list));
-			if (ret < 0)
-				break;
-
-			ret = 0;
+			if (ret >= 0)
+				ret = mutex_lock_interruptible(&file_priv->event_read_lock);
+			if (ret)
+				return ret;
 		} else {
 			unsigned length = e->event->length;
 
@@ -537,6 +544,7 @@ put_back_event:
 			e->destroy(e);
 		}
 	}
+	mutex_unlock(&file_priv->event_read_lock);
 
 	return ret;
 }