linux-kernel - Re: [patch 02/10] emm: notifier logic

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Message-ID: <Pine.LNX.4.64.0804062246030.18148@schroedinger.engr.sgi.com>
Date:	Sun, 6 Apr 2008 22:48:56 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Robin Holt <holt@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 02/10] emm: notifier logic

On Sat, 5 Apr 2008, Andrea Arcangeli wrote:

> > +	rcu_assign_pointer(mm->emm_notifier, e);
> > +	mm_unlock(mm);
> 
> My mm_lock solution makes all rcu serialization an unnecessary
> overhead so you should remove it like I already did in #v11. If it
> wasn't the case, then mm_lock wouldn't be a definitive fix for the
> race.

There still could be junk in the cache of one cpu. If you just read the 
new pointer but use the earlier content pointed to then you have a 
problem.

So a memory fence / barrier is needed to guarantee that the contents 
pointed to are fetched after the pointer.

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