Re: [PATCH] x86/refcount: Implement fast refcount_t handling

Kees Cook
2017-04-24T20:19:40Z

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