blk-mq: Fix a race between bt_clear_tag() and bt_get()

What we need is the following two guarantees:
* Any thread that observes the effect of the test_and_set_bit() by
  __bt_get_word() also observes the preceding addition of 'current'
  to the appropriate wait list. This is guaranteed by the semantics
  of the spin_unlock() operation performed by prepare_and_wait().
  Hence the conversion of test_and_set_bit_lock() into
  test_and_set_bit().
* The wait lists are examined by bt_clear() after the tag bit has
  been cleared. clear_bit_unlock() guarantees that any thread that
  observes that the bit has been cleared also observes the store
  operations preceding clear_bit_unlock(). However,
  clear_bit_unlock() does not prevent that the wait lists are examined
  before that the tag bit is cleared. Hence the addition of a memory
  barrier between clear_bit() and the wait list examination.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robert Elliott <elliott@hp.com>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: <stable@vger.kernel.org> # v3.13+
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq-tag.c

@@ -158,7 +158,7 @@ restart:
 			return -1;
 		}
 		last_tag = tag + 1;
-	} while (test_and_set_bit_lock(tag, &bm->word));
+	} while (test_and_set_bit(tag, &bm->word));
 
 	return tag;
 }
@@ -357,11 +357,10 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
 	struct bt_wait_state *bs;
 	int wait_cnt;
 
-	/*
+	clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);
-	 * The unlock memory barrier need to order access to req in free
+
-	 * path and clearing tag bit
+	/* Ensure that the wait list checks occur after clear_bit(). */
-	 */
+	smp_mb();
-	clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word);
 
 	bs = bt_wake_ptr(bt);
 	if (!bs)