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Date: Mon, 24 Apr 2017 13:19:24 -0700
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Eric Biggers <ebiggers3@...il.com>, 
	Christoph Hellwig <hch@...radead.org>, "axboe@...nel.dk" <axboe@...nel.dk>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, 
	Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>, 
	David Windsor <dwindsor@...il.com>, "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jann Horn <jann@...jh.net>, 
	"David S. Miller" <davem@...emloft.net>, linux-arch <linux-arch@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling

On Mon, Apr 24, 2017 at 3:45 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
>> +static __always_inline __must_check bool refcount_inc_not_zero(refcount_t *r)
>> +{
>> +     const int a = 1;
>> +     const int u = 0;
>> +     int c, old;
>> +
>> +     c = atomic_read(&(r->refs));
>> +     for (;;) {
>> +             if (unlikely(c == (u)))
>> +                     break;
>> +             old = atomic_cmpxchg(&(r->refs), c, c + (a));
>
> Please use atomic_try_cmpxchg(), that generates saner code.

Ah-ha, thanks. I actually copied this directly out of the existing
atomic_t function, so we should probably update it there too.

-Kees

>
>> +             if (likely(old == c))
>> +                     break;
>> +             c = old;
>> +     }
>> +     return c != u;
>> +}



-- 
Kees Cook
Pixel Security

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