linux-kernel - Re: [PATCH rcu/urgent 0/6] Fixes for RCU/scheduler/irq-threads trainwreck

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Date:	Wed, 20 Jul 2011 13:33:00 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ben Greear <greearb@...delatech.com>,
	Ed Tomlinson <edt@....ca>, linux-kernel@...r.kernel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	edward.tomlinson@...o.bombardier.com
Subject: Re: [PATCH rcu/urgent 0/6] Fixes for RCU/scheduler/irq-threads
 trainwreck

On Wed, Jul 20, 2011 at 09:57:42PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > If my guess is correct, then the minimal non-RCU_BOOST fix is #4 
> > > (which drags along #3) and #6.  Which are not one-liners, but 
> > > somewhat smaller:
> > > 
> > >  b/kernel/rcutree_plugin.h |   12 ++++++------
> > >  b/kernel/softirq.c        |   12 ++++++++++--
> > >  kernel/rcutree_plugin.h   |   31 +++++++++++++++++++++++++------
> > >  3 files changed, 41 insertions(+), 14 deletions(-)
> > 
> > That's half the patch size and half the patch count.
> > 
> > PeterZ's question is relevant: since we apparently had similar bugs 
> > in v2.6.39 as well, what changed in v3.0 that makes them so urgent 
> > to fix?
> > 
> > If it's just better instrumentation that proves them better then 
> > i'd suggest fixing this in v3.1 and not risking v3.0 with an 
> > unintended side effect.
> 
> Ok, i looked some more at the background and the symptoms that people 
> are seeing: kernel crashes and lockups. I think we want these 
> problems fixed in v3.0, even if it was the recent introduction of 
> RCU_BOOST that made it really prominent.
> 
> Having put some testing into your rcu/urgent branch today i'd feel 
> more comfortable with taking this plus perhaps an RCU_BOOST disabling 
> patch. That makes it all fundamentally tested by a number of people 
> (including those who reported/reproduced the problems).

RCU_BOOST is currently default=n.  Is that sufficient?  If not, one
low-risk approach would be for me to just remove RCU_BOOST from init/Kconfig
in 3.0 and add it back in for 3.1.  Please let me know what works best.

							Thanx, Paul

> Linus, would that approach be fine with you? I'll send an RFC pull 
> request for the 6 patches as a reply to this mail, in a couple of 
> minutes.
> 
> Thanks,
> 
> 	Ingo
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