linux-kernel - Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log

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Date:	Sun, 12 Feb 2012 09:41:33 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	tony.luck@...el.com, tglx@...utronix.de, mingo@...e.hu,
	hpa@...or.com, x86@...nel.org, linux-edac@...r.kernel.org,
	linux-kernel@...r.kernel.org, marcos.mage@...il.com,
	prasad@...ux.vnet.ibm.com
Subject: Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer()

On Mon, Feb 06, 2012 at 05:18:02PM +0100, Borislav Petkov wrote:
> On Fri, Jan 13, 2012 at 01:44:35PM -0800, Paul E. McKenney wrote:
> > Looks good to me, but I do need to defer to people who know this code
> > better than do I.  The key thing that (from what I can see) makes
> > rcu_dereference() unnecessary is that the smp_rmb() used in conjunction
> > with polling the .finished field takes care of ordering.
> 
> Right, this was me trying hard not to screw up touching mcelog.next,
> thus trying to use the rcu_dereference_index_check() primitive without
> thinking it through too much. But you're right, I'm polling the
> ->finished field 4 times (totally arbitrary, btw) which should suffice
> while the mce_log() routine above writes those entries.
> 
> Although, the question still remains, since mce_log() accesses
> mcelog.next through the rcu_dereference_index_check() primitive,
> shouldn't I do it the same way?

I don't claim to be an mce_log() expert, but when I looked it over,
I didn't see a need for rcu_dereference_index_check().  Unless I am
confused (quite possible), the memory barriers are sufficient.

The rcu_read_lock() and rcu_read_unlock() seem to be needed to avoid
premature freeing, though.

							Thanx, Paul

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