linux-kernel - Re: 2.6.26-rc4: RIP __call_for_each

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Message-ID: <20080528115814.GT25504@kernel.dk>
Date:	Wed, 28 May 2008 13:58:15 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Fabio Checconi <fchecconi@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>, torvalds@...l.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.26-rc4: RIP __call_for_each_cic+0x20/0x50

On Wed, May 28 2008, Fabio Checconi wrote:
> > From: Jens Axboe <jens.axboe@...cle.com>
> > Date: Wed, May 28, 2008 12:07:21PM +0200
> >
> > On Tue, May 27 2008, Paul E. McKenney wrote:
> > > o	When calling cfq_slab_kill(), for example from cfq_exit(),
> > > 	what ensures that all previous RCU callbacks have completed?
> > > 	
> > > 	I suspect that you need an rcu_barrier() at the beginning
> > > 	of cfq_slab_kill(), but I could be missing something.
> > 
> > So we have two callers of that, one is from the error path at init time
> > and is obviously ok. The other does need rcu_barrier()! I'll add that.
> > 
> 
> But isn't the ioc_gone completion (notified only when there are no more
> cic allocated) assuring that cfq_slab_kill() is called only after all
> the rcu callbacks are completed?  This should avoid the need for the
> rcu_barrier().

Good point, I was thinking we decremented the mod count on call_rcu(),
but we don't actually do it before the rcu callback has completed. So
that part is actually OK already.

-- 
Jens Axboe

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