linux-kernel - Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"

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Date:	Tue, 24 May 2011 17:16:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage
 based on semi-formal proof"

On Tue, May 24, 2011 at 05:05:30PM -0700, Paul E. McKenney wrote:
> On Tue, May 24, 2011 at 02:23:45PM -0700, Yinghai Lu wrote:
> > On 05/23/2011 06:35 PM, Paul E. McKenney wrote:

[ . . . ]

> > after:
> > commit bcd6e68330f893a81b3519ab3c5fc2bebbc9988c
> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Date:   Tue Sep 7 10:38:22 2010 -0700
> > 
> >     rcu: Decrease memory-barrier usage based on semi-formal proof
> > ...
> > 
> > got:
> > 
> > [   32.447936] cpu_dev_init done
> > [  111.027066] memory_dev_init done
> 
> So there is something nasty in this patch.
> 
> Not seeing it immediately, but it does give me some focus for both
> code inspection and possible diagnostic patches.

Actually, I already do have some debugfs stuff that should help me spot
the problem.

So could you please build both with and without this commit enabling
CONFIG_TRACE_RCU and send me the contents of the debugfs files rcu/rcuhier
and rcu/rcudata in both cases?

This will show me the results of the full boot path.  If this turns out to
drown out the differences, I will create a more focused diagnostic patch.

							Thanx, Paul
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