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

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Date:	Mon, 1 Feb 2016 22:53:53 +0100
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 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_ */
>