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

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Message-ID: <20110720190141.GJ2313@linux.vnet.ibm.com>
Date:	Wed, 20 Jul 2011 12:01:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Ben Greear <greearb@...delatech.com>,
	Linus Torvalds <torvalds@...ux-foundation.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 08:52:58PM +0200, Peter Zijlstra wrote:
> On Wed, 2011-07-20 at 20:44 +0200, Ingo Molnar wrote:
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > RCU boost is new and counts as strange for another few days at least.
> > 
> > I'd rather prefer a oneliner patch that turns it off in the Kconfig 
> > for now.
> > 
> > We are awfully late in the -rc cycle and this diffstat:
> > 
> >  b/include/linux/sched.h   |    3 ++
> >  b/kernel/rcutree_plugin.h |    6 +++--
> >  b/kernel/sched.c          |   44 ++++++++++++++++++++++++++++++++++-----
> >  b/kernel/signal.c         |   19 +++++++++++------
> >  b/kernel/softirq.c        |   12 +++++++++-
> >  kernel/rcutree_plugin.h   |   51 +++++++++++++++++++++++++++++++++-------------
> >  6 files changed, 105 insertions(+), 30 deletions(-)
> > 
> > looks very scary to me.
> 
> A lot of that is also relevant to !BOOST.

Unfortunately, agreed.  :-(

The expedited grace periods also cause rcu_read_unlock() to call
into the scheduler.  This can interact badly with the recently
added RCU read-side critical sections in the scheduler that have
either the runqueue or the priority-inheritance locks held, especially
when interrupts occur towards the end of __rcu_read_unlock().  But
there are some failure cases that don't involve interrupts (though
I believe all of them involve RCU_BOOST).

That said, turning off RCU_BOOST would allow dropping this patches:

7765be2fec0f4 Fix RCU_BOOST race handling current->rcu_read_unlock_special



							Thanx, Paul
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