Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

[Linus Torvalds <torvalds@osdl.org>]

On Sat, 21 Oct 2006, Nick Piggin wrote:
> 
> So moving the flush_cache_mm below the copy_page_range, to just
> before the flush_tlb_mm, would work then? This would make the
> race much smaller than with this patchset.
> 
> But doesn't that still leave a race?
> 
> What if another thread writes to cache after we have flushed it
> but before flushing the TLBs? Although we've marked the the ptes
> readonly, the CPU won't trap if the TLB is valid? There must be
> some special way for the arch to handle this, but I can't see it.

Why not do the cache flush _after_ the TLB flush? There's still a mapping, 
and never mind that it's read-only: the _mapping_ still exists, and I 
doubt any CPU will not do the writeback (the readonly bit had better 
affect the _frontend_ of the memory pipeline, but affectign the back end 
would be insane and very hard, since you can't raise a fault any more).

Hmm?

		Linus
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