linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Date:	Mon, 11 Apr 2011 12:59:23 -0400
From:	"hch@...radead.org" <hch@...radead.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Mike Snitzer <snitzer@...hat.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>, dm-devel@...hat.com,
	linux-raid@...r.kernel.org
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On Mon, Apr 11, 2011 at 02:50:22PM +1000, NeilBrown wrote:
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 273d60b..903ce8d 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2674,19 +2674,23 @@ static void flush_plug_list(struct blk_plug *plug)
>  	struct request_queue *q;
>  	unsigned long flags;
>  	struct request *rq;
> +	struct list_head head;
>  
>  	BUG_ON(plug->magic != PLUG_MAGIC);
>  
>  	if (list_empty(&plug->list))
>  		return;
> +	list_add(&head, &plug->list);
> +	list_del_init(&plug->list);
>  
>  	if (plug->should_sort)
> -		list_sort(NULL, &plug->list, plug_rq_cmp);
> +		list_sort(NULL, &head, plug_rq_cmp);
> +	plug->should_sort = 0;

As Jens mentioned this should be list_splice_init.  But looking over
flush_plug_list the code there seems strange to me.

What does the local_irq_save in flush_plug_list protect?  Why don't
we need it over the list_sort?  And do we still need it when first
splicing the list to a local one?

It's one of these cases where I'd really like to see more comments
explaining why the code is doing what it's doing.

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