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

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Date:	Thu, 01 Apr 2010 17:50:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Avi Kivity <avi@...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, 2010-04-01 at 17:42 +0200, 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
> 
> I personally prefer read-write locks that Christoph used for both of
> them, but I'm not against mutex either. Still the refcounting problem
> should be the same as it's introduced by allowing the critical
> sections under anon_vma->lock to schedule (no matter if it's mutex or
> read-write sem).

Right, so the problem with the rwsem is that, esp for very short hold
times, they introduce more pain than they're worth. Also the rwsem
doesn't do adaptive spinning nor allows for lock stealing, resulting in
a much much heavier sync. object than the mutex is.

You also seem to move the tlb_gather stuff around, we have patches in
-rt that make tlb_gather preemptible, once i_mmap_lock is preemptible we
can do in mainline too.



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