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

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Message-ID: <20100426144542.GA2529@linux.vnet.ibm.com>
Date:	Mon, 26 Apr 2010 07:45:42 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Li Zefan <lizf@...fujitsu.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 09:39:20AM -0400, Vivek Goyal wrote:
> On Sun, Apr 25, 2010 at 07:06:31PM -0700, Paul E. McKenney wrote:
> > 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.
> 
> Thanks Paul.
> 
> I sent this patch to Jens also, thinking he will apply to his tree. Looks
> like he has not applied it yet though.
> 
> Jens, is it ok if this patch gets merged through paul's tree or should it
> go through blk tree?

I am happy for it to go either way, so just let me know!

							Thanx, Paul
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