Re: [PATCH 2/9] block: Stop abusing rq->csd.list in blk-softirq

[Frederic Weisbecker <fweisbec@gmail.com>]

Hi Jan,

I'm currently working on some cleanups on IPI code too and working on top
of these patches, just have a few comments:

On Mon, Dec 23, 2013 at 09:39:23PM +0100, Jan Kara wrote:
> Abusing rq->csd.list for a list of requests to complete is rather ugly.
> Especially since using queuelist should be safe and much cleaner.

It would be nice to have a few more details that explain why doing so is safe
wrt a block request lifecycle. At least something that tells why rq->queuelist
can't be ever used concurrently by the time we send the IPI and we trigger/raise
the softirq.

> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  block/blk-softirq.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/block/blk-softirq.c b/block/blk-softirq.c
> index 57790c1a97eb..7ea5534096d5 100644
> --- a/block/blk-softirq.c
> +++ b/block/blk-softirq.c
> @@ -30,8 +30,8 @@ static void blk_done_softirq(struct softirq_action *h)
>  	while (!list_empty(&local_list)) {
>  		struct request *rq;
>  
> -		rq = list_entry(local_list.next, struct request, csd.list);
> -		list_del_init(&rq->csd.list);
> +		rq = list_entry(local_list.next, struct request, queuelist);
> +		list_del_init(&rq->queuelist);
>  		rq->q->softirq_done_fn(rq);
>  	}
>  }
> @@ -45,9 +45,9 @@ static void trigger_softirq(void *data)
>  
>  	local_irq_save(flags);
>  	list = this_cpu_ptr(&blk_cpu_done);
> -	list_add_tail(&rq->csd.list, list);
> +	list_add_tail(&rq->queuelist, list);

And given that's an alternate use of rq->queuelist, perhaps add a comment
to unconfuse people.

Thanks.

>  
> -	if (list->next == &rq->csd.list)
> +	if (list->next == &rq->queuelist)
>  		raise_softirq_irqoff(BLOCK_SOFTIRQ);
>  
>  	local_irq_restore(flags);
> @@ -136,7 +136,7 @@ void __blk_complete_request(struct request *req)
>  		struct list_head *list;
>  do_local:
>  		list = this_cpu_ptr(&blk_cpu_done);
> -		list_add_tail(&req->csd.list, list);
> +		list_add_tail(&req->queuelist, list);
>  
>  		/*
>  		 * if the list only contains our just added request,
> @@ -144,7 +144,7 @@ do_local:
>  		 * entries there, someone already raised the irq but it
>  		 * hasn't run yet.
>  		 */
> -		if (list->next == &req->csd.list)
> +		if (list->next == &req->queuelist)
>  			raise_softirq_irqoff(BLOCK_SOFTIRQ);
>  	} else if (raise_blk_irq(ccpu, req))
>  		goto do_local;
> -- 
> 1.8.1.4
>