block: protect nr_requests update using q->elevator_lock

The sysfs attribute nr_requests could be simultaneously updated from
elevator switch/update or nr_hw_queue update code path. The update to
nr_requests for each of those code paths runs holding q->elevator_lock.
So we should protect access to sysfs attribute nr_requests using q->
elevator_lock instead of q->sysfs_lock.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250304102551.2533767-6-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 1562e22877e14fe903d8143e4b8a69efa2f0826e..f1fa57de29edeb45a0516a3b241ca26c8e6de1f7 100644 (file)
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -55,9 +55,9 @@ static ssize_t queue_requests_show(struct gendisk *disk, char *page)
 {
        ssize_t ret;
 
-       mutex_lock(&disk->queue->sysfs_lock);
+       mutex_lock(&disk->queue->elevator_lock);
        ret = queue_var_show(disk->queue->nr_requests, page);
-       mutex_unlock(&disk->queue->sysfs_lock);
+       mutex_unlock(&disk->queue->elevator_lock);
        return ret;
 }
 
@@ -76,16 +76,16 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
        if (ret < 0)
                return ret;
 
-       mutex_lock(&q->sysfs_lock);
        memflags = blk_mq_freeze_queue(q);
+       mutex_lock(&q->elevator_lock);
        if (nr < BLKDEV_MIN_RQ)
                nr = BLKDEV_MIN_RQ;
 
        err = blk_mq_update_nr_requests(disk->queue, nr);
        if (err)
                ret = err;
+       mutex_unlock(&q->elevator_lock);
        blk_mq_unfreeze_queue(q, memflags);
-       mutex_unlock(&q->sysfs_lock);
        return ret;
 }
 
@@ -692,7 +692,6 @@ static struct attribute *blk_mq_queue_attrs[] = {
        /*
         * Attributes which are protected with q->sysfs_lock.
         */
-       &queue_requests_entry.attr,
 #ifdef CONFIG_BLK_WBT
        &queue_wb_lat_entry.attr,
 #endif
@@ -701,6 +700,7 @@ static struct attribute *blk_mq_queue_attrs[] = {
         * q->sysfs_lock.
         */
        &elv_iosched_entry.attr,
+       &queue_requests_entry.attr,
 
        /*
         * Attributes which don't require locking.

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 31b1b635c7101c3707cbb7698f20d6ef5557d74b..3e66ad016a230fb39111a369f2a0f467a73a500f 100644 (file)
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -561,10 +561,12 @@ struct request_queue {
        struct list_head        flush_list;
 
        /*
-        * Protects against I/O scheduler switching, specifically when
-        * updating q->elevator. 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, this lock also protects the sysfs
+        * attribute nr_requests.
+        * To ensure proper locking order during an elevator update, first
+        * freeze the queue, then acquire ->elevator_lock.
         */
        struct mutex            elevator_lock;