linux-kernel - Re: [PATCH] kref: prefer atomic_inc_not_zero to atomic_add

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Message-ID: <CAHmME9rVPQ+SWqAe6Lqj4Do16mpCmSUz5_6Q2qyYj6mqnJec7g@mail.gmail.com>
Date:	Thu, 30 Jun 2016 00:52:02 +0200
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	Thomas Hellstrom <thellstrom@...are.com>,
	Dave Airlie <airlied@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless

This was positively reviewed by maintainers but never picked up. Can
someone queue this for 4.7 or 4.8?

Thanks,
Jason

On Mon, Feb 1, 2016 at 10:53 PM, Jason A. Donenfeld <Jason@...c4.com> wrote:
> This was positively reviewed but never picked up. Can someone queue
> this for rc3?
>
> Thanks,
> Jason
>
> On Sun, Oct 11, 2015 at 9:59 PM, Thomas Hellstrom <thellstrom@...are.com> wrote:
>> Reviewed-by: Thomas Hellstrom <thellstrom@...are.com>
>>
>>
>> On 10/10/2015 12:56 PM, Jason A. Donenfeld wrote:
>>> On most platforms, there exists this ifdef:
>>>
>>>  #define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
>>>
>>> This makes this patch functionally useless. However, on PPC, there is
>>> actually an explicit definition of atomic_inc_not_zero with its own
>>> assembly that is slightly more optimized than atomic_add_unless. So,
>>> this patch changes kref to use atomic_inc_not_zero instead, for PPC and
>>> any future platforms that might provide an explicit implementation.
>>>
>>> This also puts this usage of kref more in line with a verbatim reading
>>> of the examples in Paul McKenney's paper [1] in the section titled "2.4
>>> Atomic Counting With Check and Release Memory Barrier", which uses
>>> atomic_inc_not_zero.
>>>
>>> [1] https://urldefense.proofpoint.com/v2/url?u=http-3A__open-2Dstd.org_jtc1_sc22_wg21_docs_papers_2007_n2167.pdf&d=BQIBAg&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=z5Nd9sYiJMKiphNjyZp6XT5CbayXMBlcb903f260pDY&s=HEHX3CuXRs2GRRQWuC4Vef6iJMwdilKVRkiZgJpjEpA&e=
>>>
>>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
>>> ---
>>>  include/linux/kref.h | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/kref.h b/include/linux/kref.h
>>> index 484604d..83d1f94 100644
>>> --- a/include/linux/kref.h
>>> +++ b/include/linux/kref.h
>>> @@ -166,6 +166,6 @@ static inline int kref_put_mutex(struct kref *kref,
>>>   */
>>>  static inline int __must_check kref_get_unless_zero(struct kref *kref)
>>>  {
>>> -     return atomic_add_unless(&kref->refcount, 1, 0);
>>> +     return atomic_inc_not_zero(&kref->refcount);
>>>  }
>>>  #endif /* _KREF_H_ */
>>