linux-kernel - Re: [PATCH] mm: numa: Guarantee that tlb_flush_pending updates are visible before page table updates

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Date:	Wed, 11 Dec 2013 16:40:52 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Alex Thorlton <athorlton@....com>,
	Rik van Riel <riel@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: numa: Guarantee that tlb_flush_pending updates are
 visible before page table updates

On Wed, Dec 11, 2013 at 06:44:47AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 11, 2013 at 01:21:09PM +0000, Mel Gorman wrote:
> > According to documentation on barriers, stores issued before a LOCK can
> > complete after the lock implying that it's possible tlb_flush_pending can
> > be visible after a page table update. As per revised documentation, this patch
> > adds a smp_mb__before_spinlock to guarantee the correct ordering.
> > 
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> 
> Assuming that there is a lock acquisition after calls to
> set_tlb_flush_pending():
> 
> Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> (I don't see set_tlb_flush_pending() in mainline.)
> 

It's introduced by a patch flight that is currently sitting in Andrew's
tree. In the case where we care about the value of tlb_flush_pending, a
spinlock will be taken. PMD or PTE split spinlocks or the mm->page_table_lock
depending on whether it is 3.13 or 3.12-stable and earlier kernels. I
pushed the relevant patches to this tree and branch

git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux-balancenuma.git numab-instrument-serialise-v5r1

There is no guarantee the lock will be taken if there are no pages populated
in the region but we also do not care about flushing the TLB in that case
either. Does it matter that there is no guarantee a lock will be taken
after smp_mb__before_spinlock, just very likely that it will be?

-- 
Mel Gorman
SUSE Labs
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