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>

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 3feeeccf8a992e360625aa8b0e9156eb297e63cb..24656980f4431a76b025e196e66d88f0950b5512 100644 (file)
--- a/block/blk-mq-sysfs.c
+++ b/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;
 }
 

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5a2d6392752529da0e8f7b2b8b6855e1f6cf8ad9..b9550a127c8e0b337cb9388b144ca0a2af2aae12 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -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);
 }
 
 /*

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3bee1b4858b6f42802c3d94628bc46d6eb78caf5..dcf8fce15e23dd1ffeefe74426bceb2c59d255b1 100644 (file)
--- 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;