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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 24 Apr 2019 09:29:50 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x3O8Tnk432112772 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 24 Apr 2019 08:29:49 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 28867B2067; Wed, 24 Apr 2019 08:29:49 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 06C55B2065; Wed, 24 Apr 2019 08:29:49 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.142.230]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 24 Apr 2019 08:29:48 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 49A5016C1A95; Wed, 24 Apr 2019 01:29:48 -0700 (PDT) Date: Wed, 24 Apr 2019 01:29:48 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Nicholas Piggin , LKMM Maintainers -- Akira Yokosawa , Andrea Parri , Boqun Feng , David Howells , Daniel Lustig , Jade Alglave , Kernel development list , Luc Maranget , Alan Stern , Will Deacon Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic() Reply-To: paulmck@linux.ibm.com References: <20190419180017.GP4038@hirez.programming.kicks-ass.net> <20190419182620.GF14111@linux.ibm.com> <1555719429.t9n8gkf70y.astroid@bobo.none> <20190420085440.GK14111@linux.ibm.com> <20190423121715.GQ4038@hirez.programming.kicks-ass.net> <20190423132116.GJ3923@linux.ibm.com> <20190423132620.GU4038@hirez.programming.kicks-ass.net> <20190423201637.GW3923@linux.ibm.com> <20190423202831.GA4038@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190423202831.GA4038@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19042408-0064-0000-0000-000003D0370D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010985; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000285; SDB=6.01193611; UDB=6.00625732; IPR=6.00974435; MB=3.00026572; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-24 08:29:53 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19042408-0065-0000-0000-00003D2F725F Message-Id: <20190424082948.GG3923@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-24_06:,, 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=856 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904240073 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 23, 2019 at 10:28:31PM +0200, Peter Zijlstra wrote: > On Tue, Apr 23, 2019 at 01:16:37PM -0700, Paul E. McKenney wrote: > > > Agreed, but I thought that one of the ideas going forward was to get > > rid of smp_mb__{before,after}_atomic(). > > It's not one I had considered.. I just wanted to get rid of this > 'surprise' behaviour. Ah, good point, your patch is in fact a midpoint between those two positions. Just to make sure I understand: 1. Without your patch, smp_mb__{before,after}_atomic() orders only against the atomic itself. 2. With your patch, smp_mb__{before,after}_atomic() orders against the atomic itself and the accesses on the other side of that atomic. However, it does not order the atomic against the accesses on the other side of that atomic. Putting things between the smp_mb__{before,after}_atomic() and the atomic is in my opinion a bad idea, but in this case they are not necessarily ordered. 3. Dispensing with smp_mb__{before,after}_atomic() would have void RMW atomics fully ordered, but I suspect that it results in ugly performance regressions. Or am I still missing something? Thanx, Paul