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

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Message-ID: <20120217170713.GA26575@google.com>
Date:	Fri, 17 Feb 2012 09:07:13 -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

Hey, Vivek.

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.

> >  {
> >  	struct blkio_group *blkg, *new_blkg;
> >  
> > -	WARN_ON_ONCE(!rcu_read_lock_held());
> 
> Don't we want to be called with rcu lock held needed for blkg_lookup()?

We want WARN_ON_ONCE(none of RCU, blkcg and queue lock held).  We can
do it using lockdep macros inside #ifdef CONFIG_LOCKDEP.  It's just a
bit clunky.

Thanks.

-- 
tejun
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