Re: [PATCH] mm: prevent mmap_cache race in find_vma()

[Hugh Dickins]

On Thu, 4 Apr 2013, Linus Torvalds wrote:
> On Thu, Apr 4, 2013 at 11:35 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > find_vma() can be called by multiple threads with read lock
> > held on mm->mmap_sem and any of them can update mm->mmap_cache.
> > Prevent compiler from re-fetching mm->mmap_cache, because other
> > readers could update it in the meantime:
> 
> Ack. I do wonder if we should mark the unlocked update too some way
> (also in find_vma()), although it's probably not a problem in practice
> since there's no way the compiler can reasonably really do anything
> odd with it. We *could* make that an ACCESS_ONCE() write too just to
> highlight the fact that it's an unlocked write to this optimistic data
> structure.

Hah, you beat me to it.

I wanted to get Jan's patch in first, seeing as it actually fixes his
observed issue; and it is very nice to have such a good description of
one of those, when ACCESS_ONCE() is usually just an insurance policy.

But then I was researching the much rarer "ACCESS_ONCE(x) = y" usage
(popular in drivers/net/wireless/ath/ath9k and kernel/rcutree* and
sound/firewire, but few places else).

When Paul reminded us of it yesterday, I came to wonder if actually
every use of ACCESS_ONCE in the read form should strictly be matched
by ACCESS_ONCE whenever modifying the location.

My uneducated guess is that strictly it ought to, in the sense of
insurance policy; but that (apart from that strange split writing
issue which came up a couple of months ago) in practice our compilers
have not "advanced" to the point of making this an issue yet.

> 
> Anyway, applied.

Thanks,
Hugh
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