Re: CFQ is broken for CONFIG_BLK_CGROUP=y, CFQ_GROUP_IOSCHED=n

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Apr 28, 2010 at 07:36:02PM +0400, Dmitry Monakhov wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
> 
> > On Wed, Apr 28, 2010 at 04:44:51PM +0400, Dmitry Monakhov wrote:
> >> 
> >> I've had an  oops on kernel boot due to NULL pointer deference
> >> linux-2.6-block/for-next HEAD:7eaed1226ab411ee5dc8c34fc0d8034e4c98e3c6
> >> I've enabled CONFIG_BLK_CGROUP, but not CFQ_GROUP_IOSCHED
> >> In this case cfq_ref_get_cfqg() defined as 
> >> static inline struct cfq_group *cfq_ref_get_cfqg(struct cfq_group *cfqg)
> >> {
> >>         return NULL;
> >> }
> >> So following call trace is simply NOOP
> >> cfq_set_request()
> >>  rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
> >> 
> >> Which later result in OOPS on bio insertion
> >> cfq_insert_request
> >>  -> blkiocg_update_io_add_stats(&(RQ_CFQG(rq))->blkg,...)
> >>     -> spin_lock_irqsave(&blkg->stats_lock, flags);
> >> 
> >> Bad commit.
> >> >From 7f1dc8a2d2f45fc557b27fd56115338b1d34fc24 Mon Sep 17 00:00:00 2001
> >> From: Vivek Goyal <vgoyal@redhat.com>
> >> Date: Wed, 21 Apr 2010 17:44:16 +0200
> >> Subject: [PATCH] blkio: Fix blkio crash during rq stat update
> >
> > Dmitry, this patch should fix the issue. Can you please give it a try.
> Yes. the issue is gone.
> >
> > Jens, I know you don't like this form of cfq_ref_get_cfqg(), but this
> > seems to be the simplest solution to fix it.
> IMHO following fix is simpler, and cleaner.

> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 286008c..0f3eb70 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1094,7 +1094,7 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd, int create)
>  
>  static inline struct cfq_group *cfq_ref_get_cfqg(struct cfq_group *cfqg)
>  {
> -	return NULL;
> +	return cfqg;
>  }

Thanks Dmitry. I like it. simpler.

Vivek