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

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Message-ID: <20110526011310.GP2341@linux.vnet.ibm.com>
Date:	Wed, 25 May 2011 18:13:10 -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 Wed, May 25, 2011 at 03:49:25PM -0700, Yinghai Lu wrote:
> On 05/25/2011 03:34 PM, Paul E. McKenney wrote:
> > On Wed, May 25, 2011 at 03:15:50PM -0700, Yinghai Lu wrote:
> >>> There is a new branch yinghai.2011.05.24a on:
> >>>
> >>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git
> >>>
> >>> Or will be as soon as kernel.org updates its mirrors.
> >>>
> >>> I am not sure I could call this "clean", but it does revert that commit
> >>> and 11 of the subsequent commits that depend on it.  It does build,
> >>> and I will test it once my currently running tests complete.
> >>
> >> yes, with those revert, there is no delay in 10 times booting.
> > 
> > Unfortunately, there are rcutorture test failures with the revert...
> 
> confused.

Given what I had to do to generate the revert, not exactly a surprise,
I am afraid.  Just means that the resulting RCU sometimes fails to
wait for all pre-existing readers, and rcutorture catches it.

> what is the next?

1.	I send you a patch that I hope will fix the softlockup
	you saw.  I am testing this.

2.	I am working on more detailed instrumentation, and will
	send a patch on that.

3.	If time allows, break down the operations RCU is doing
	and test them in isolation.

Other thoughts?

							Thanx, Paul
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