Re: [PATCH 4/4] block: keep q_usage_counter in atomic mode after del_gendisk

[Bart Van Assche <bvanassche@xxxxxxx>]

On 9/20/21 04:24, Christoph Hellwig wrote:
> Don't switch back to percpu mode to avoid the double RCU grace period
> when tearing down SCSI devices.  After removing the disk only passthrough
> commands can be send anyway.
                  ^^^^
                  sent?

> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 108a352051be5..bc026372de439 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -188,9 +188,11 @@ void blk_mq_freeze_queue(struct request_queue *q)
>   }
>   EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);
>   
> -void blk_mq_unfreeze_queue(struct request_queue *q)
> +void __blk_mq_unfreeze_queue(struct request_queue *q, bool force_atomic)
>   {
>   	mutex_lock(&q->mq_freeze_lock);
> +	if (force_atomic)
> +		q->q_usage_counter.data->force_atomic = true;
>   	q->mq_freeze_depth--;
>   	WARN_ON_ONCE(q->mq_freeze_depth < 0);
>   	if (!q->mq_freeze_depth) {
> @@ -199,6 +201,11 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
>   	}
>   	mutex_unlock(&q->mq_freeze_lock);
>   }
> +
> +void blk_mq_unfreeze_queue(struct request_queue *q)
> +{
> +	__blk_mq_unfreeze_queue(q, false);
> +}
>   EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
>   
>   /*
> diff --git a/block/blk.h b/block/blk.h
> index e2ed2257709ae..6c3c00a8fe19d 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -51,6 +51,7 @@ struct blk_flush_queue *blk_alloc_flush_queue(int node, int cmd_size,
>   void blk_free_flush_queue(struct blk_flush_queue *q);
>   
>   void blk_freeze_queue(struct request_queue *q);
> +void __blk_mq_unfreeze_queue(struct request_queue *q, bool force_atomic);
>   void blk_queue_start_drain(struct request_queue *q);
>   
>   #define BIO_INLINE_VECS 4
> diff --git a/block/genhd.c b/block/genhd.c
> index b3c33495d7208..fe23085ddddd6 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -596,6 +596,7 @@ void del_gendisk(struct gendisk *disk)
>   	/*
>   	 * Allow using passthrough request again after the queue is torn down.
>   	 */
> +	blk_queue_flag_clear(QUEUE_FLAG_INIT_DONE, q);
>   	blk_mq_unfreeze_queue(q);
>   
>   	if (!(disk->flags & GENHD_FL_HIDDEN)) {

I don't see any code that passes 'true' as second argument to
__blk_mq_unfreeze_queue()? Did I perhaps overlook something?

Thanks,

Bart.