linux-kernel - Re: [PATCH 12/12] cciss: fix for iostat

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Date:	Mon, 6 Nov 2006 21:45:51 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Mike Miller (OS Dev)" <mikem@...rdog.cca.cpqcorp.net>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH 12/12] cciss: fix for iostat

On Mon, Nov 06 2006, Mike Miller (OS Dev) wrote:
> Patch 12 of 12
> 
> This patch replaces complete_buffers with end_that_request_first to fix
> programs like iostat. This has been broken for the last few kernel releases.
> Please consider this for inclusion.
> 
> Thanks,
> mikem
> 
> Signed-off-by: Mike Miller <mike.miller@...com>
> 
> 
> ---
> 
> 
> ---
> 
>  drivers/block/cciss.c |   20 ++++----------------
>  1 files changed, 4 insertions(+), 16 deletions(-)
> 
> diff -puN drivers/block/cciss.c~cciss_update_diskstats_fix drivers/block/cciss.c
> --- linux-2.6/drivers/block/cciss.c~cciss_update_diskstats_fix	2006-11-06 13:28:53.000000000 -0600
> +++ linux-2.6-root/drivers/block/cciss.c	2006-11-06 13:28:53.000000000 -0600
> @@ -1156,18 +1156,6 @@ static int cciss_ioctl(struct inode *ino
>  	}
>  }
>  
> -static inline void complete_buffers(struct bio *bio, int status)
> -{
> -	while (bio) {
> -		struct bio *xbh = bio->bi_next;
> -		int nr_sectors = bio_sectors(bio);
> -
> -		bio->bi_next = NULL;
> -		bio_endio(bio, nr_sectors << 9, status ? 0 : -EIO);
> -		bio = xbh;
> -	}
> -}
> -
>  static void cciss_check_queues(ctlr_info_t *h)
>  {
>  	int start_queue = h->next_to_run;
> @@ -1236,15 +1224,15 @@ static void cciss_softirq_done(struct re
>  		pci_unmap_page(h->pdev, temp64.val, cmd->SG[i].Len, ddir);
>  	}
>  
> -	complete_buffers(rq->bio, rq->errors);
> -
>  #ifdef CCISS_DEBUG
>  	printk("Done with %p\n", rq);
>  #endif				/* CCISS_DEBUG */
>  
> -	add_disk_randomness(rq->rq_disk);
>  	spin_lock_irqsave(&h->lock, flags);
> -	end_that_request_last(rq, rq->errors);
> +	if (!end_that_request_first(rq, rq->errors, rq->nr_sectors)) {
> +		add_disk_randomness(rq->rq_disk);
> +		end_that_request_last(rq, rq->errors);
> +	}
>  	cmd_free(h, cmd, 1);
>  	cciss_check_queues(h);
>  	spin_unlock_irqrestore(&h->lock, flags);

Ah, so there's where that went. Your code isn't clear, though -
end_that_request_first() _must_ return 0, so the above looks confusing.
It would look cleaner and more informative like:

        if (end_that_request_first(rq, rq->errors, rq->nr_sectors))
                BUG();

        add_disk_randomness(rq->rq_disk);
        end_that_request_last(rq, rq->errors);
        ...

and so on. Additionally you don't need the lock for
end_that_request_first(), so it's a lot more optimal to rearrange it
again.

        add_disk_randomness(rq->rq_disk);
        if (end_that_request_first(rq, rq->errors, rq->nr_sectors))
                BUG();

        spin_lock_irqsave(&h->lock, flags);
        end_that_request_last(rq, rq->errors);
        cmd_free(h, cmd, 1);
        ...

Not only cleaner to read since it's obvious what will happen, also moves
the heavy path (end_that_request_first()) outside of the controller
lock.

-- 
Jens Axboe

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