linux-kernel - Re: [PATCH 06/17] arm: mmu

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Message-ID: <20110228151547.GC4911@flint.arm.linux.org.uk>
Date:	Mon, 28 Feb 2011 15:15:47 +0000
From:	Russell King <rmk@....linux.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	"Luck,Tony" <tony.luck@...el.com>, PaulMundt <lethal@...ux-sh.org>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [PATCH 06/17] arm: mmu_gather rework

On Mon, Feb 28, 2011 at 04:05:48PM +0100, Peter Zijlstra wrote:
> On Mon, 2011-02-28 at 14:57 +0000, Russell King wrote:
> > On Mon, Feb 28, 2011 at 03:18:47PM +0100, Peter Zijlstra wrote:
> > > On Mon, 2011-02-28 at 12:44 +0100, Peter Zijlstra wrote:
> > > >   unmap_region()
> > > >     tlb_gather_mmu()
> > > >     unmap_vmas()
> > > >       for (; vma; vma = vma->vm_next)
> > > >         unmao_page_range()
> > > >           tlb_start_vma() -> flush cache range
> > > 
> > > So why is this correct? Can't we race with a concurrent access to the
> > > memory region (munmap() vs other thread access race)? While
> > > unmap_region() callers will have removed the vma from the tree so faults
> > > will not be satisfied, TLBs might still be present and allow us to
> > > access the memory and thereby reloading it in the cache.
> > 
> > It is my understanding that code sections between tlb_gather_mmu() and
> > tlb_finish_mmu() are non-preemptible - that was the case once upon a
> > time when this stuff first appeared.  
> 
> It is still so, but that doesn't help with SMP. The case mentioned above
> has two threads running, one doing munmap() and the other is poking at
> the memory being unmapped.

Luckily its a no-op on SMP capable CPUs (and actually is also a no-op
on any PIPT or VIPT ARM CPU.)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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