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Date: Mon, 24 Apr 2017 13:16:24 -0700
From: Kees Cook <>
To: Peter Zijlstra <>
Cc: LKML <>, Eric Biggers <>, 
	Christoph Hellwig <>, "" <>, 
	James Bottomley <>, 
	Elena Reshetova <>, Hans Liljestrand <>, 
	David Windsor <>, "" <>, Ingo Molnar <>, 
	Arnd Bergmann <>, Greg Kroah-Hartman <>, Jann Horn <>, 
	"David S. Miller" <>, linux-arch <>, 
	"" <>, PaX Team <>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 1:32 AM, Peter Zijlstra <> wrote:
> On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
>> This patch ports the x86-specific atomic overflow handling from PaX's
>> PAX_REFCOUNT to the upstream refcount_t API. This is an updated version
>> from PaX that eliminates the saturation race condition by resetting the
>> atomic counter back to the INT_MAX saturation value on both overflow and
>> underflow. To win a race, a system would have to have INT_MAX threads
>> simultaneously overflow before the saturation handler runs.
> And is this impossible? Highly unlikely I'll grant you, but absolutely
> impossible?

I'll adjust the language. "Highly unlikely" is still better than
"trivially doable with a single thread". :)

> Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> O(nr_tasks + 3*nr_cpus).
> Because PaX does it, is not a correctness argument. And this really
> wants one.

Sure, I didn't mean to imply anything other than a demonstration of
what PaX is doing (and that it's better than not having it). If we can
improve it, that's great.


Kees Cook
Pixel Security

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