linux-kernel - Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init

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Message-ID: <20100426020631.GU2440@linux.vnet.ibm.com>
Date:	Sun, 25 Apr 2010 19:06:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] blk-cgroup: Fix RCU correctness warning in
 cfq_init_queue()

On Mon, Apr 26, 2010 at 09:33:46AM +0800, Li Zefan wrote:
> >>>>>> With RCU correctness on, We see following warning. This patch fixes it.
> >>>>> This is in initialization code, so that there cannot be any concurrent
> >>>>> updates, correct?  If so, looks good.
> >>>>>
> >>>> I think theoritically two instances of cfq_init_queue() can be running
> >>>> in parallel (for two different devices), and they both can call
> >>>> blkiocg_add_blkio_group(). But then we use a spin lock to protect
> >>>> blkio_cgroup.
> >>>>
> >>>> spin_lock_irqsave(&blkcg->lock, flags);
> >>>>
> >>>> So I guess two parallel updates should be fine.
> >>> OK, in that case, would it be possible add this spinlock to the condition
> >>> checked by css_id()'s rcu_dereference_check()?
> >> Hi Paul,
> >>
> >> I think adding these spinlock to condition checked might become little
> >> messy. And the reason being that this lock is subsystem (controller)
> >> specific and maintained by controller. Now if any controller implements
> >> a lock and we add that lock in css_id() rcu_dereference_check(), it will
> >> look ugly.
> >>
> >> So probably a better way is to make sure that css_id() is always called
> >> under rcu read lock so that we don't hit this warning?
> > 
> > As long as holding rcu_read_lock() prevents css_id() from the usual
> > problems such as access memory that was concurrently freed, yes.
> 
> blkiocg_add_blkio_group() also calls cgroup_path(), which also needs to
> be called within rcu_read_lock, so I think Vivek's patch is better than
> the one you posted in another mail thread.

My apologies, Vivek!  I lost track of your patch.  I have now replaced
my patch with yours.

							Thanx, Paul
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