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

[Hugh Dickins <hugh@veritas.com>]

On Wed, 30 Apr 2008, Benjamin Herrenschmidt wrote:
> On Tue, 2008-04-29 at 13:36 +0100, Hugh Dickins wrote:
> > 
> > Ugh.  It's just so irritating to introduce these blockages against
> > such a remote possibility (but there again, that's what so much of
> > kernel code has to be about).  Is there any other way of handling it?
> 
> Not that much overhead... I think smp_read_barrier_depends() is a nop on
> most archs no ? The data dependency between all the pointers takes care
> of ordering in many cases.

Ah, you're right, I was automatically thinking smp_rmb, whereas as this
is the only_does_something_on_alpha_mb (nice to see those impressively
long comments on a "do { } while (0)" in some of the other arches ;)

(Well, frv says "barrier()" for it - does it actually need that?)

Yes, that's not bad at all; though in that case,
I am surprised it's enough to patch up the issue.

> So it boils down to smp_wmb's when setting
> which is not that expensive.

Yes, I wasn't worried about the much less common and anyway heavier
write (allocate) path, I don't begrudge the smp_wmb's there.

Thanks for calming me down!
Hugh

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