blk-mq-tag: check for NULL rq when iterating tags

Since we introduced blk-mq-sched, the tags->rqs[] array has been
dynamically assigned. So we need to check for NULL when iterating,
since there's a window of time where the bit is set, but we haven't
dynamically assigned the tags->rqs[] array position yet.

This is perfectly safe, since the memory backing of the request is
never going away while the device is alive.
Reviewed-by: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq-tag.c

@@ -214,7 +214,11 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 		bitnr += tags->nr_reserved_tags;
 	rq = tags->rqs[bitnr];
 
-	if (rq->q == hctx->queue)
+	/*
+	 * We can hit rq == NULL here, because the tagging functions
+	 * test and set the bit before assining ->rqs[].
+	 */
+	if (rq && rq->q == hctx->queue)
 		iter_data->fn(hctx, rq, iter_data->data, reserved);
 	return true;
 }
@@ -248,9 +252,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 
 	if (!reserved)
 		bitnr += tags->nr_reserved_tags;
+
+	/*
+	 * We can hit rq == NULL here, because the tagging functions
+	 * test and set the bit before assining ->rqs[].
+	 */
 	rq = tags->rqs[bitnr];
+	if (rq)
+		iter_data->fn(rq, iter_data->data, reserved);
 
-	iter_data->fn(rq, iter_data->data, reserved);
 	return true;
 }