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

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Message-ID: <20110720195742.GA14671@elte.hu>
Date:	Wed, 20 Jul 2011 21:57:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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


* 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).

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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