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

JIRA: https://issues.redhat.com/browse/RHEL-112997

commit 5abba4c
Author: Nilay Shroff <nilay@linux.ibm.com>
Date:   Thu Mar 6 15:09:53 2025 +0530

    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: Nilay 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: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-mq-sysfs.c

@@ -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

@@ -4095,6 +4095,8 @@ 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;
@@ -4199,6 +4201,8 @@ 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

@@ -580,12 +580,14 @@ 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;