blk-cgroup: delay calling blkcg_exit_disk until disk_release

While del_gendisk ensures there is no outstanding I/O on the queue,
it can't prevent block layer users from building new I/O.

This leads to a NULL ->root_blkg reference in bio_associate_blkg when
allocating a new bio on a shut down file system.  Delay freeing the
blk-cgroup subsystems from del_gendisk until disk_release to make
sure the blkg and throttle information is still avaіlable for bio
submitters, even if those bios will immediately fail.

This now can cause a case where disk_release is called on a disk
that hasn't been added.  That's mostly harmless, except for a case
in blk_throttl_exit that now needs to check for a NULL ->td pointer.

Fixes: 178fa7d49815 ("blk-cgroup: delay blk-cgroup initialization until add_disk")
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230208063514.171485-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 902203b..e7bd705 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2411,7 +2411,8 @@ void blk_throtl_exit(struct gendisk *disk)
 {
        struct request_queue *q = disk->queue;
 
-       BUG_ON(!q->td);
+       if (!q->td)
+               return;
        del_timer_sync(&q->td->service_queue.pending_timer);
        throtl_shutdown_wq(q);
        blkcg_deactivate_policy(disk, &blkcg_policy_throtl);

diff --git a/block/genhd.c b/block/genhd.c
index 7e03155..6537373 100644 (file)
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -668,8 +668,6 @@ void del_gendisk(struct gendisk *disk)
        rq_qos_exit(q);
        blk_mq_unquiesce_queue(q);
 
-       blkcg_exit_disk(disk);
-
        /*
         * If the disk does not own the queue, allow using passthrough requests
         * again.  Else leave the queue frozen to fail all I/O.
@@ -1166,6 +1164,8 @@ static void disk_release(struct device *dev)
        might_sleep();
        WARN_ON_ONCE(disk_live(disk));
 
+       blkcg_exit_disk(disk);
+
        /*
         * To undo the all initialization from blk_mq_init_allocated_queue in
         * case of a probe failure where add_disk is never called we have to