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>

1 files changed, 8 insertions, 6 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3bee1b4858b6..dcf8fce15e23 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -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;