From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751543AbbJKT7l (ORCPT ); Sun, 11 Oct 2015 15:59:41 -0400 Received: from smtp-outbound-2.vmware.com ([208.91.2.13]:52536 "EHLO smtp-outbound-2.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751094AbbJKT7j (ORCPT ); Sun, 11 Oct 2015 15:59:39 -0400 Subject: Re: [PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless To: "Jason A. Donenfeld" , Dave Airlie , References: <1444474594-28359-1-git-send-email-Jason@zx2c4.com> From: Thomas Hellstrom Message-ID: <561ABFA6.8050102@vmware.com> Date: Sun, 11 Oct 2015 21:59:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <1444474594-28359-1-git-send-email-Jason@zx2c4.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.113.160.246] X-ClientProxiedBy: EX13-CAS-013.vmware.com (10.113.191.65) To EX13-MBX-024.vmware.com (10.113.191.44) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Reviewed-by: Thomas Hellstrom 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 > --- > 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_ */