blk-mq: provide a helper to check if a queue is busy

Returns true if the queue currently has requests pending,
false if not.

DM can use this to replace the atomic_inc/dec they do per device
to see if a device is busy.
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -790,6 +790,32 @@ struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
 }
 EXPORT_SYMBOL(blk_mq_tag_to_rq);
 
+static bool blk_mq_check_busy(struct blk_mq_hw_ctx *hctx, struct request *rq,
+			      void *priv, bool reserved)
+{
+	/*
+	 * If we find a request, we know the queue is busy. Return false
+	 * to stop the iteration.
+	 */
+	if (rq->q == hctx->queue) {
+		bool *busy = priv;
+
+		*busy = true;
+		return false;
+	}
+
+	return true;
+}
+
+bool blk_mq_queue_busy(struct request_queue *q)
+{
+	bool busy = false;
+
+	blk_mq_queue_tag_busy_iter(q, blk_mq_check_busy, &busy);
+	return busy;
+}
+EXPORT_SYMBOL_GPL(blk_mq_queue_busy);
+
 static void blk_mq_rq_timed_out(struct request *req, bool reserved)
 {
 	req->rq_flags |= RQF_TIMED_OUT;

include/linux/blk-mq.h

@@ -250,6 +250,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule);
 void blk_mq_free_request(struct request *rq);
 bool blk_mq_can_queue(struct blk_mq_hw_ctx *);
 
+bool blk_mq_queue_busy(struct request_queue *q);
+
 enum {
 	/* return when out of requests */
 	BLK_MQ_REQ_NOWAIT	= (__force blk_mq_req_flags_t)(1 << 0),