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 08:56:20 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mel Gorman <mgorman@...e.de>
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 04:40:52PM +0000, Mel Gorman wrote:
> 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?

If you do smp_mb__before_spinlock() without a lock acquisition, no harm
will be done, other than possibly a bit of performance loss.  So you
should be OK.

							Thanx, Paul

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