block: don't allow to switch elevator if updating nr_hw_queues is in-progress

Elevator switch code is another `nr_hw_queue` reader in non-fast-IO code
path, so it can't be done if updating `nr_hw_queues` is in-progress.

Take same approach with not allowing add/del disk when updating
nr_hw_queues is in-progress, by grabbing read lock of
set->update_nr_hwq_sema.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/linux-block/aAWv3NPtNIKKvJZc@fedora/
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-10-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/elevator.c b/block/elevator.c
index 4400eb8fe54f1ec55d2082f048f51da292eec51b..2e18513dcd739196b7eb7b02bb0400dbd719acad 100644 (file)
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -723,6 +723,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
        int ret;
        unsigned int memflags;
        struct request_queue *q = disk->queue;
+       struct blk_mq_tag_set *set = q->tag_set;
 
        /*
         * If the attribute needs to load a module, do it before freezing the
@@ -734,6 +735,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 
        elv_iosched_load_module(name);
 
+       down_read(&set->update_nr_hwq_lock);
        memflags = blk_mq_freeze_queue(q);
        mutex_lock(&q->elevator_lock);
        ret = elevator_change(q, name);
@@ -741,6 +743,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
                ret = count;
        mutex_unlock(&q->elevator_lock);
        blk_mq_unfreeze_queue(q, memflags);
+       up_read(&set->update_nr_hwq_lock);
        return ret;
 }