linux-kernel - Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all

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Date:	Thu, 1 Apr 2010 09:14:18 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in
 mmu_take_all_locks()

On Thu, Apr 01, 2010 at 05:56:46PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-04-01 at 18:51 +0300, Avi Kivity wrote:
> > On 04/01/2010 06:42 PM, Andrea Arcangeli wrote:
> > > On Thu, Apr 01, 2010 at 01:43:14PM +0200, Peter Zijlstra wrote:
> > >    
> > >> On Thu, 2010-04-01 at 13:27 +0200, Peter Zijlstra wrote:
> > >>      
> > >>> I've almost got a patch done that converts those two, still need to look
> > >>> where that tasklist_lock muck happens.
> > >>>        
> > >> OK, so the below builds and boots, only need to track down that
> > >> tasklist_lock nesting, but I got to run an errand first.
> > >>      
> > > You should have a look at my old patchset where Christoph already
> > > implemented this (and not for decreasing latency but to allow
> > > scheduling in mmu notifier handlers, only needed by XPMEM):
> > >
> > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/
> > >
> > > The ugliest part of it (that I think you missed below) is the breakage
> > > of the RCU locking in the anon-vma which requires adding refcounting
> > > to it. That was the worst part of the conversion as far as I can tell.
> > >
> > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/anon-vma
> > >    
> > 
> > Can we use srcu now instead?
> 
> I would much rather we make call_rcu_preempt() available at all times.

Even in !CONFIG_PREEMPT kernels?  Or am I missing your point?

							Thanx, Paul
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