dm snapshot: fix lockup in dm_exception_table_exit

[ Upstream commit 6e7132ed3c07bd8a6ce3db4bb307ef2852b322dc ]

There was reported lockup when we exit a snapshot with many exceptions.
Fix this by adding "cond_resched" to the loop that frees the exceptions.

Reported-by: John Pittman <jpittman@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/md/dm-snap.c

@@ -686,8 +686,10 @@ static void dm_exception_table_exit(struct dm_exception_table *et,
 	for (i = 0; i < size; i++) {
 		slot = et->table + i;
 
-		hlist_bl_for_each_entry_safe(ex, pos, n, slot, hash_list)
+		hlist_bl_for_each_entry_safe(ex, pos, n, slot, hash_list) {
 			kmem_cache_free(mem, ex);
+			cond_resched();
+		}
 	}
 
 	kvfree(et->table);