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

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Message-ID: <20120206161802.GC31237@aftab>
Date:	Mon, 6 Feb 2012 17:18:02 +0100
From:	Borislav Petkov <bp@...64.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	bp@...64.org, 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 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?

Thanks.

-- 
Regards/Gruss,
Boris.

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