Commit 5abba4ce authored by Nilay Shroff's avatar Nilay Shroff Committed by Jens Axboe
Browse files

block: protect hctx attributes/params using q->elevator_lock 



Currently, hctx attributes (nr_tags, nr_reserved_tags, and cpu_list)
are protected using `q->sysfs_lock`. However, these attributes can be
updated in multiple scenarios:
  - During the driver's probe method.
  - When updating nr_hw_queues.
  - When writing to the sysfs attribute nr_requests,
    which can modify nr_tags.
The nr_requests attribute is already protected using q->elevator_lock,
but none of the update paths actually use q->sysfs_lock to protect hctx
attributes. So to ensure proper synchronization, replace q->sysfs_lock
with q->elevator_lock when reading hctx attributes through sysfs.

Additionally, blk_mq_update_nr_hw_queues allocates and updates hctx.
The allocation of hctx is protected using q->elevator_lock, however,
updating hctx params happens without any protection, so safeguard hctx
param update path by also using q->elevator_lock.

Signed-off-by: default avatarNilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250306093956.2818808-1-nilay@linux.ibm.com


[axboe: wrap comment at 80 chars]
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 5e40f445

block/blk-mq-sysfs.c

+2 −2
Original line number Diff line number Diff line
@@ -61,9 +61,9 @@ static ssize_t blk_mq_hw_sysfs_show(struct kobject *kobj, 
	if (!entry->show)

		return -EIO;



	mutex_lock(&q->sysfs_lock);

	mutex_lock(&q->elevator_lock);

	res = entry->show(hctx, page);

	mutex_unlock(&q->sysfs_lock);

	mutex_unlock(&q->elevator_lock);

	return res;

}

block/blk-mq.c

+4 −0
Original line number Diff line number Diff line
@@ -4094,6 +4094,8 @@ static void blk_mq_map_swqueue(struct request_queue *q) 
	struct blk_mq_ctx *ctx;

	struct blk_mq_tag_set *set = q->tag_set;



	mutex_lock(&q->elevator_lock);



	queue_for_each_hw_ctx(q, hctx, i) {

		cpumask_clear(hctx->cpumask);

		hctx->nr_ctx = 0;
@@ -4198,6 +4200,8 @@ static void blk_mq_map_swqueue(struct request_queue *q) 
		hctx->next_cpu = blk_mq_first_mapped_cpu(hctx);

		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;

	}



	mutex_unlock(&q->elevator_lock);

}



/*

include/linux/blkdev.h

+8 −6
Original line number Diff line number Diff line
@@ -561,12 +561,14 @@ struct request_queue { 
	struct list_head	flush_list;



	/*

	 * Protects against I/O scheduler switching, particularly when

	 * updating q->elevator. Since the elevator update code path may

	 * also modify q->nr_requests and wbt latency, this lock also

	 * protects the sysfs attributes nr_requests and wbt_lat_usec.

	 * To ensure proper locking order during an elevator update, first

	 * freeze the queue, then acquire ->elevator_lock.

	 * Protects against I/O scheduler switching, particularly when updating

	 * q->elevator. Since the elevator update code path may also modify q->

	 * nr_requests and wbt latency, this lock also protects the sysfs attrs

	 * nr_requests and wbt_lat_usec. Additionally the nr_hw_queues update

	 * may modify hctx tags, reserved-tags and cpumask, so this lock also

	 * helps protect the hctx attrs. To ensure proper locking order during

	 * an elevator or nr_hw_queue update, first freeze the queue, then

	 * acquire ->elevator_lock.

	 */

	struct mutex		elevator_lock;