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

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Message-ID: <Pine.LNX.4.64.0610200846260.3962@g5.osdl.org>
Date:	Fri, 20 Oct 2006 08:49:35 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	David Miller <davem@...emloft.net>, ralf@...ux-mips.org,
	akpm@...l.org, linux-kernel@...r.kernel.org, anemo@....ocn.ne.jp
Subject: Re: [PATCH 1/3] Fix COW D-cache aliasing on fork



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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