linux-kernel - Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

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Message-ID: <20120217171428.GA29414@google.com>
Date:	Fri, 17 Feb 2012 09:14:28 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

On Fri, Feb 17, 2012 at 09:07:13AM -0800, Tejun Heo wrote:
> On Fri, Feb 17, 2012 at 11:19:58AM -0500, Vivek Goyal wrote:
> > On Thu, Feb 16, 2012 at 02:37:51PM -0800, Tejun Heo wrote:
> > > Now that blkg additions / removals are always done under both q and
> > > blkcg locks, the only place RCU locking is used is blkg_lookup() for
> > > lockless lookup.  This patch drops unncessary RCU locking replacing it
> > > with plain blkcg / q locking as necessary.
> > > 
> > > * blkg_lookup_create() and blkiocg_pre_destroy() already perform
> > >   proper locking and don't need RCU.  Dropped.
> > 
> > But blkg_lookup_create() is called under rcu() to protect blkcg pointer.
> > And blkg_lookup() is also happening under same rcu read lock. So I think
> > you can't drop rcu from blkg_lookup_create().
> 
> Ooh, right.  Will fix.

BTW, the reason this doesn't work is due to the broken percpu
allocation in blkg_alloc().  We should remove unnecessary rcu lockings
after updating stat allocation.

Thanks.

-- 
tejun
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