linux-kernel - Re: [PATCH] mm: prevent mmap_cache race in find

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Date:	Thu, 4 Apr 2013 12:10:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Stancek <jstancek@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ian Lance Taylor <iant@...gle.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: prevent mmap_cache race in find_vma()

On Thu, Apr 4, 2013 at 12:01 PM, Hugh Dickins <hughd@...gle.com> wrote:
>
> 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.

I don't see how a compiler could reasonably really ever do anything
different, but I do think the ACCESS_ONCE() modification version might
be a good thing just as a "documentation".

This is a good example of this issue, exactly because we have a mix of
both speculative cases (the find_vma() lookup and modification)
together with strictly exclusive locked accesses to the same field
(the ones that invalidate the cache under the write lock). So
documenting that the write in find_vma() is this kind of "optimistic
unlocked access" is actually a potentially interesting piece of
information for programmers, completely independently of whether the
compiler will then treat it really differently or not.

Of course, a plain comment would do the same, but would be less greppable.

And despite the verbiage here, I don't really have a very strong
opinion on this. I'm going to let it go, and if somebody sends me a
patch with a good explanation in the next merge window, I'll probably
apply it.

                    Linus
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