Hi Bart,
Sorry for the delayed response since I was on holiday.
On 18/2/10 02:44, Bart Van Assche wrote:
> Avoid that the following race can occur:
>
> blk_cleanup_queue() blkcg_print_blkgs()
> spin_lock_irq(lock) (1) spin_lock_irq(blkg->q->queue_lock) (2,5)
> q->queue_lock = &q->__queue_lock (3)
> spin_unlock_irq(lock) (4)
> spin_unlock_irq(blkg->q->queue_lock) (6)
>
> (1) take driver lock;
> (2) busy loop for driver lock;
> (3) override driver lock with internal lock;
> (4) unlock driver lock;
> (5) can take driver lock now;
> (6) but unlock internal lock.
>
> This change is safe because only the SCSI core and the NVME core keep
> a reference on a request queue after having called blk_cleanup_queue().
> Neither driver accesses any of the removed data structures between its
> blk_cleanup_queue() and blk_put_queue() calls.
>
> Reported-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx>
> Cc: Jan Kara <jack@xxxxxxxx>
> ---
> block/blk-core.c | 31 +++++++++++++++++++++++++++++++
> block/blk-sysfs.c | 7 -------
> 2 files changed, 31 insertions(+), 7 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 41c74b37be85..6febc69a58aa 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -719,6 +719,37 @@ void blk_cleanup_queue(struct request_queue *q)
> del_timer_sync(&q->backing_dev_info->laptop_mode_wb_timer);
> blk_sync_queue(q);
>
> + /*
> + * I/O scheduler exit is only safe after the sysfs scheduler attribute
> + * has been removed.
> + */
> + WARN_ON_ONCE(q->kobj.state_in_sysfs);
> +
I notice that several devices such as loop and zram will call
blk_cleanup_queue before del_gendisk, so it will hit this warning. Is
this normal?
Thanks,
Joseph
> + /*
> + * Since the I/O scheduler exit code may access cgroup information,
> + * perform I/O scheduler exit before disassociating from the block
> + * cgroup controller.
> + */
> + if (q->elevator) {
> + ioc_clear_queue(q);
> + elevator_exit(q, q->elevator);
> + q->elevator = NULL;
> + }
> +
> + /*
> + * Remove all references to @q from the block cgroup controller before
> + * restoring @q->queue_lock to avoid that restoring this pointer causes
> + * e.g. blkcg_print_blkgs() to crash.
> + */
> + blkcg_exit_queue(q);
> +
> + /*
> + * Since the cgroup code may dereference the @q->backing_dev_info
> + * pointer, only decrease its reference count after having removed the
> + * association with the block cgroup controller.
> + */
> + bdi_put(q->backing_dev_info);
> +
> if (q->mq_ops)
> blk_mq_free_queue(q);
> percpu_ref_exit(&q->q_usage_counter);
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index cbea895a5547..fd71a00c9462 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -798,13 +798,6 @@ static void __blk_release_queue(struct work_struct *work)
> if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
> blk_stat_remove_callback(q, q->poll_cb);
> blk_stat_free_callback(q->poll_cb);
> - bdi_put(q->backing_dev_info);
> - blkcg_exit_queue(q);
> -
> - if (q->elevator) {
> - ioc_clear_queue(q);
> - elevator_exit(q, q->elevator);
> - }
>
> blk_free_queue_stats(q->stats);
>
>
