drm/nouveau/ltc: protect clearing of comptags with mutex

BugLink: http://bugs.launchpad.net/bugs/1655041

commit f4e65efc upstream.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/gpu/drm/nouveau/nvkm/subdev/ltc/base.c

@@ -47,8 +47,10 @@ nvkm_ltc_tags_clear(struct nvkm_ltc *ltc, u32 first, u32 count)
 
 	BUG_ON((first > limit) || (limit >= ltc->num_tags));
 
+	mutex_lock(&ltc->subdev.mutex);
 	ltc->func->cbc_clear(ltc, first, limit);
 	ltc->func->cbc_wait(ltc);
+	mutex_unlock(&ltc->subdev.mutex);
 }
 
 int