blk-mq: clarify dispatch may not be drained/blocked by stopping queue

BLK_MQ_S_STOPPED may not be observed in other concurrent I/O paths,
we can't guarantee that dispatching won't happen after returning
from the APIs of stopping queue.

So clarify the fact and avoid potential misuse.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <Bart.VanAssche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1204,6 +1204,15 @@ static void __blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx, bool sync)
 	set_bit(BLK_MQ_S_STOPPED, &hctx->state);
 }
 
+/*
+ * This function is often used for pausing .queue_rq() by driver when
+ * there isn't enough resource or some conditions aren't satisfied, and
+ * BLK_MQ_RQ_QUEUE_BUSY is usually returned.
+ *
+ * We do not guarantee that dispatch can be drained or blocked
+ * after blk_mq_stop_hw_queue() returns. Please use
+ * blk_mq_quiesce_queue() for that requirement.
+ */
 void blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
 	__blk_mq_stop_hw_queue(hctx, false);
@@ -1219,6 +1228,15 @@ static void __blk_mq_stop_hw_queues(struct request_queue *q, bool sync)
 		__blk_mq_stop_hw_queue(hctx, sync);
 }
 
+/*
+ * This function is often used for pausing .queue_rq() by driver when
+ * there isn't enough resource or some conditions aren't satisfied, and
+ * BLK_MQ_RQ_QUEUE_BUSY is usually returned.
+ *
+ * We do not guarantee that dispatch can be drained or blocked
+ * after blk_mq_stop_hw_queues() returns. Please use
+ * blk_mq_quiesce_queue() for that requirement.
+ */
 void blk_mq_stop_hw_queues(struct request_queue *q)
 {
 	__blk_mq_stop_hw_queues(q, false);