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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 27 May 2019 20:23:53 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4RJNqu439387206 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 27 May 2019 19:23:52 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C6B9CB2067; Mon, 27 May 2019 19:23:52 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A48C9B2064; Mon, 27 May 2019 19:23:52 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.199.73]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 27 May 2019 19:23:52 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id D347016C396E; Mon, 27 May 2019 12:23:56 -0700 (PDT) Date: Mon, 27 May 2019 12:23:56 -0700 From: "Paul E. McKenney" To: Joe Perches Cc: Andrea Parri , linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org, Peter Zijlstra , Will Deacon , Mark Rutland , Matthew Wilcox , Sasha Levin , apw@canonical.com Subject: Re: [PATCH v2] rcu: Don't return a value from rcu_assign_pointer() Reply-To: paulmck@linux.ibm.com References: <1558946997-25559-1-git-send-email-andrea.parri@amarulasolutions.com> <20190527161050.GK28207@linux.ibm.com> <810a0dae47c90c39015903c413303fcee89ab5eb.camel@perches.com> <20190527174901.GL28207@linux.ibm.com> <475b18e78ba7e04363fe002315f75d2ce35496f9.camel@perches.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <475b18e78ba7e04363fe002315f75d2ce35496f9.camel@perches.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19052719-0072-0000-0000-00000433DAAD X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011172; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01209443; UDB=6.00635346; IPR=6.00990469; MB=3.00027076; MTD=3.00000008; XFM=3.00000015; UTC=2019-05-27 19:23:58 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19052719-0073-0000-0000-00004C62F272 Message-Id: <20190527192356.GN28207@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-05-27_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905270136 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 27, 2019 at 10:57:52AM -0700, Joe Perches wrote: > On Mon, 2019-05-27 at 10:49 -0700, Paul E. McKenney wrote: > > On Mon, May 27, 2019 at 10:21:22AM -0700, Joe Perches wrote: > > > On Mon, 2019-05-27 at 09:10 -0700, Paul E. McKenney wrote: > > > > On Mon, May 27, 2019 at 10:49:57AM +0200, Andrea Parri wrote: > > > > > Quoting Paul [1]: > > > > > > > > > > "Given that a quick (and perhaps error-prone) search of the uses > > > > > of rcu_assign_pointer() in v5.1 didn't find a single use of the > > > > > return value, let's please instead change the documentation and > > > > > implementation to eliminate the return value." > > > > > > > > > > [1] https://lkml.kernel.org/r/20190523135013.GL28207@linux.ibm.com > > > > > > > > Queued, thank you! > > > > > > > > Adding the checkpatch maintainers on CC as well. The "do { } while > > > > (0)" prevents the return value from being used, by design. Given the > > > > checkpatch complaint, is there some better way to achieve this? > > > > > > Not sure what the checkpatch complaint is here. > > > > Checkpatch seems to want at least two statements in each > > "do { } while (0)" macro definition: > > > > WARNING: Single statement macros should not use a do {} while (0) loop > > > > > Reading the link above, there seems to be a compiler warning. > > > > The compiler warning is a theoretical issue that is being fixed by this > > patch, and the patch is giving the checkpatch warning. > > > > > Perhaps a statement expression macro with no return value? > > > > > > #define rcu_assign_pointer(p, v) ({ (p) = (v); ; }) > > > > This is at best an acquired taste for me... > > Another ugly possibility could be: > > #define rcu_assign_pointer(p, v) do {if (1) (p) = (v); } while (0) And, not to be left out, another ugly possibility might be: #define rcu_assign_pointer(p, v) ((void)((p) = (v))) > Possibly the best option would be to ignore checkpatch here > and just add a comment above the use. Works for me! Thanx, Paul