linux-kernel - [PATCH for-6.18/block 04/10] blk-mq: convert to serialize updating nr_requests with update_nr_hwq

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Message-Id: <20250908061533.3062917-5-yukuai1@huaweicloud.com>
Date: Mon,  8 Sep 2025 14:15:27 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: nilay@...ux.ibm.com,
	ming.lei@...hat.com,
	axboe@...nel.dk
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yukuai3@...wei.com,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: [PATCH for-6.18/block 04/10] blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock

From: Yu Kuai <yukuai3@...wei.com>

request_queue->nr_requests can be changed by:

a) switching elevator by update nr_hw_queues
b) switching elevator by elevator sysfs attribute
c) configue queue sysfs attribute nr_requests

Current lock order is:

1) update_nr_hwq_lock, case a,b
2) freeze_queue
3) elevator_lock, cas a,b,c

And update nr_requests is seriablized by elevator_lock() already,
however, in the case c), we'll have to allocate new sched_tags if
nr_requests grow, and do this with elevator_lock held and queue
freezed has the risk of deadlock.

Hence use update_nr_hwq_lock instead, make it possible to allocate
memory if tags grow, meanwhile also prevent nr_requests to be changed
concurrently.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/blk-sysfs.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f99519f7a820..7ea15bf68b4b 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -68,13 +68,14 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
 	int ret, err;
 	unsigned int memflags;
 	struct request_queue *q = disk->queue;
+	struct blk_mq_tag_set *set = q->tag_set;
 
 	ret = queue_var_store(&nr, page, count);
 	if (ret < 0)
 		return ret;
 
-	memflags = blk_mq_freeze_queue(q);
-	mutex_lock(&q->elevator_lock);
+	/* serialize updating nr_requests with switching elevator */
+	down_write(&set->update_nr_hwq_lock);
 
 	if (nr == q->nr_requests)
 		goto unlock;
@@ -89,13 +90,18 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
 		goto unlock;
 	}
 
+	memflags = blk_mq_freeze_queue(q);
+	mutex_lock(&q->elevator_lock);
+
 	err = blk_mq_update_nr_requests(disk->queue, nr);
 	if (err)
 		ret = err;
 
-unlock:
 	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, memflags);
+
+unlock:
+	up_write(&set->update_nr_hwq_lock);
 	return ret;
 }
 
-- 
2.39.2