[dm-devel] [for-4.16 PATCH v4 2/4] block: use queue_lock when clearing QUEUE_FLAG_REGISTERED in blk_unregister_queue

[Ming Lei]

On Thu, Jan 11, 2018 at 03:14:15PM -0500, Mike Snitzer wrote:
> blk_unregister_queue() must protect against any modifications of
> q->queue_flags (not just those performed in blk-sysfs.c).  Therefore
> q->queue_lock needs to be used rather than q->sysfs_lock.
> 
> Fixes: e9a823fb34a8b ("block: fix warning when I/O elevator is changed as request_queue is being removed")
> Cc: stable at vger.kernel.org # 4.14+
> Reported-by: Bart Van Assche <Bart.VanAssche at wdc.com>
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>  block/blk-sysfs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 870484eaed1f..52f57539f1c7 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -929,9 +929,9 @@ void blk_unregister_queue(struct gendisk *disk)
>  	if (WARN_ON(!q))
>  		return;
>  
> -	mutex_lock(&q->sysfs_lock);
> +	spin_lock_irq(q->queue_lock);
>  	queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
> -	mutex_unlock(&q->sysfs_lock);
> +	spin_unlock_irq(q->queue_lock);
>  
>  	wbt_exit(q);

Hi Mike,

This change may not be correct, since at least e9a823fb34a8b depends
on q->sysfs_lock to sync between testing the flag in __elevator_change()
and clearing it here.

Thanks,
Ming