Re: [patch] mm: fix race in COW logic

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sun, 22 Jun 2008, Hugh Dickins wrote:
> 
> You have a wicked mind, and I think you're right, and the fix right.

Agreed. I think the patch is fine, although I'd personally probably like 
it even more if the mm counter updates to follow the rmap updates.

> One thing though, in moving the page_remove_rmap in that way, aren't
> you assuming that there's an appropriate wmbarrier between the two
> locations?  If that is necessarily so (there's plenty happening in
> between), it may deserve a comment to say just where that barrier is.

In this case, I don't think memory ordering matters.

What matters is that the map count never goes down to one - and by 
re-ordering the inc/dec accesses, that simply won't happen. IOW, memory 
ordering is immaterial, only the ordering of count updates (from the 
standpoint of the faulting CPU - so that's not even an SMP issue) matters.

		Linus

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