ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()

After we acquire the sma->sem_perm lock in exit_sem(), we are protected
against a racing IPC_RMID operation.  Also at that point, we are the last
user of sem_undo_list.  Therefore it isn't required that we acquire or use
ulp->lock.
Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Rafael Aquini <aquini@redhat.com>
CC: Aristeu Rozanski <aris@redhat.com>
Cc: David Jeffery <djeffery@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -2123,9 +2123,11 @@ void exit_sem(struct task_struct *tsk)
 		ipc_assert_locked_object(&sma->sem_perm);
 		list_del(&un->list_id);
 
-		spin_lock(&ulp->lock);
+		/* we are the last process using this ulp, acquiring ulp->lock
+		 * isn't required. Besides that, we are also protected against
+		 * IPC_RMID as we hold sma->sem_perm lock now
+		 */
 		list_del_rcu(&un->list_proc);
-		spin_unlock(&ulp->lock);
 
 		/* perform adjustments registered in un */
 		for (i = 0; i < sma->sem_nsems; i++) {