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

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Date:	Thu, 01 Apr 2010 17:56:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in
 mmu_take_all_locks()

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.

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