fs/epoll: robustify ep->mtx held checks

mainline inclusion
from mainline-5.0-rc1
commit 21877e1a
category: bugfix
bugzilla: 10119
CVE: NA

---------------------------

Insted of just commenting how important it is, lets make it more robust
and add a lockdep_assert_held() call.

Link: http://lkml.kernel.org/r/20181108051006.18751-5-dave@stgolabs.netSigned-off-by: NDavidlohr Bueso <dbueso@suse.de>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nzhengbin <zhengbin13@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/eventpoll.c

@@ -1638,6 +1638,8 @@ static __poll_t ep_send_events_proc(struct eventpoll *ep, struct list_head *head
 	 * Items cannot vanish during the loop because ep_scan_ready_list() is
 	 * holding "mtx" during this call.
 	 */
+	lockdep_assert_held(&ep->mtx);
+
 	for (esed->res = 0, uevent = esed->events;
 	     !list_empty(head) && esed->res < esed->maxevents;) {
 		epi = list_first_entry(head, struct epitem, rdllink);