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Date: Mon, 24 Apr 2017 10:32:50 +0200
From: Peter Zijlstra <>
To: Kees Cook <>
Cc:, Eric Biggers <>,
	Christoph Hellwig <>,
	"" <>,
	James Bottomley <>,
	Elena Reshetova <>,
	Hans Liljestrand <>,
	David Windsor <>,,
	Ingo Molnar <>, Arnd Bergmann <>,
	Greg Kroah-Hartman <>,
	Jann Horn <>,,,,
	PaX Team <>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

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

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.

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