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

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Date:	Thu, 21 Jul 2011 09:04:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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 08:25:24PM -0700, Ben Greear wrote:
> On 07/20/2011 02:12 PM, Paul E. McKenney wrote:
> >On Wed, Jul 20, 2011 at 01:54:49PM -0700, Ben Greear wrote:
> >>On 07/20/2011 01:33 PM, Paul E. McKenney wrote:
> >>>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
> >>
> >>Not if it remains broken I think..unless you put it under CONFIG_BROKEN
> >>or something.  Otherwise, folks are liable to turn it on and not realize
> >>it's the cause of subtle bugs.
> >
> >Good point, I could easily add "depends on BROKEN".
> >
> >>For what it's worth, my tests have been running clean for around 2 hours, so the full set of
> >>fixes with RCU_BOOST appears good, so far.  I'll let it continue to run
> >>at least overnight to make sure I'm not just getting lucky...
> >
> >Continuing to think good thoughts...  ;-)
> 
> My test is still going strong with no splats or errors, so I think that
> nailed the problems I was seeing...

Excellent news!!!

And again, thank you for all the testing!

							Thanx, Paul
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