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From: Kees Cook <keescook@chromium.org>
Date: Mon, 24 Apr 2017 13:19:24 -0700
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Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling
To: Peter Zijlstra <peterz@infradead.org>
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        Eric Biggers <ebiggers3@gmail.com>,
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On Mon, Apr 24, 2017 at 3:45 AM, Peter Zijlstra <peterz@infradead.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