Re: [rfc] data race in page table setup/walking?

[Nick Piggin <npiggin@suse.de>]

On Wed, Apr 30, 2008 at 12:14:51PM +0100, Hugh Dickins wrote:
> On Wed, 30 Apr 2008, Nick Piggin wrote:
> > 
> > Actually, aside, all those smp_wmb() things in pgtable-3level.h can
> > probably go away if we cared: because we could be sneaky and leverage
> > the assumption that top and bottom will always be in the same cacheline
> > and thus should be shielded from memory consistency problems :)
> 
> I've sometimes wondered along those lines.  But it would need
> interrupts disabled, wouldn't it?  And could SMM mess it up?
> And what about another CPU taking the cacheline to modify it
> in between our two accesses?

Nothing more than could not already happen with the smp_wmb in there,
AFAIKS.

 
> I don't think we do care in that x86 PAE case, but as a general
> principal, if it can be safely assumed on all architectures (or
> more messily, just on some) under certain conditions, then shouldn't
> we be looking to use that technique (relying on a consistent view of
> separate variables clustered into the same cacheline) in critical
> places, rather than regarding it as sneaky?
> 
> But I suspect this is a chimaera, that there's actually no
> safe use to be made of it.  I'd be glad to be shown wrong.

Well Linus put a dampener on it... but if it actually did work, then
yeah I guess there are some places it could be used. I suspect that
on some implementations, being in the same cacheline would actually
fully order all transactions of a CPU, so if it did make a big
difference anywhere, we could have smp_*mb_cacheline() or something ;)

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