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Date: Mon, 24 Apr 2017 13:15:53 +0200
From: Peter Zijlstra <>
To: PaX Team <>
Cc: Kees Cook <>,,
	Eric Biggers <>,
	Christoph Hellwig <>,
	"" <>,
	James Bottomley <>,
	Elena Reshetova <>,
	Hans Liljestrand <>,
	David Windsor <>,,
	Ingo Molnar <>, Arnd Bergmann <>,
	Greg Kroah-Hartman <>,
	Jann Horn <>,,,
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 01:00:18PM +0200, PaX Team wrote:
> On 24 Apr 2017 at 10:32, Peter Zijlstra wrote:

> > Also, you forgot nr_cpus in your bound. Afaict the worst case here is
> > O(nr_tasks + 3*nr_cpus).
> what does nr_cpus have to do with winning the race?

The CPUs could each run nested softirq/hardirq/nmi context poking at the
refcount, irrespective of the (preempted) task context.

> > Because PaX does it, is not a correctness argument. And this really
> > wants one.
> heh, do you want to tell me about how checking for a 0 refcount prevents
> exploiting a bug?

Not the point. All I said was that saying somebody else does it (anybody
else, doesn't matter it was you) isn't an argument for correctness.

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