Re: block: del_gendisk() vs blk_queue_enter() race condition

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/10/03 06:43), Christoph Hellwig wrote:
> .. actually, we still clear QUEUE_FLAG_DYING early.  Something like
> the pathc below (plus proper comments) should sort it out:
> 
> diff --git a/block/genhd.c b/block/genhd.c
> index 1c05dd4c6980b5..9a1e18fbb136cf 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -589,9 +589,6 @@ static void __blk_mark_disk_dead(struct gendisk *disk)
>  	if (test_and_set_bit(GD_DEAD, &disk->state))
>  		return;
>  
> -	if (test_bit(GD_OWNS_QUEUE, &disk->state))
> -		blk_queue_flag_set(QUEUE_FLAG_DYING, disk->queue);
> -
>  	/*
>  	 * Stop buffered writers from dirtying pages that can't be written out.
>  	 */
> @@ -673,6 +670,9 @@ void del_gendisk(struct gendisk *disk)
>  		drop_partition(part);
>  	mutex_unlock(&disk->open_mutex);
>  
> +	if (test_bit(GD_OWNS_QUEUE, &disk->state))
> +		blk_queue_flag_set(QUEUE_FLAG_DYING, disk->queue);

So that mutex_lock(&disk->open_mutex) right before it potentially can
deadlock (I think it will).

My idea, thus far, was to

	if (test_bit(GD_OWNS_QUEUE, &disk->state)) }
		blk_queue_flag_set(QUEUE_FLAG_DYING, disk->queue);
		blk_kick_queue_enter(disk->queue);  // this one simply wake_up_all(&q->mq_freeze_wq);
											// if the queue has QUEUE_FLAG_DYING
	}

in del_gendisk() before the very first time del_gendisk() attempts to
mutex_lock(&disk->open_mutex), because that mutex is already locked
forever.

>  	if (!(disk->flags & GENHD_FL_HIDDEN)) {
>  		sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");