From 39a70c76b89b81db91a72a86b6c6a9b239013417 Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@redhat.com>
Date: Tue, 6 Jun 2017 23:22:09 +0800
Subject: [PATCH] blk-mq: clarify dispatch may not be drained/blocked by
 stopping queue
Git-commit: 39a70c76b89b81db91a72a86b6c6a9b239013417
Patch-mainline: v4.13-rc1
References: fate#322738,fate#322919,fate#322950,fate#323773

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>
Signed-off-by: Coly Li <colyli@suse.de>

---
 block/blk-mq.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index dbae586602f6..89cbd022b1eb 100644
--- a/block/blk-mq.c
+++ b/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);
-- 
2.13.1