From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0F7E9C43381 for ; Wed, 27 Mar 2019 15:04:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DD9012082F for ; Wed, 27 Mar 2019 15:04:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729075AbfC0PEf (ORCPT ); Wed, 27 Mar 2019 11:04:35 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:54946 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728250AbfC0PEf (ORCPT ); Wed, 27 Mar 2019 11:04:35 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x2RF3FfC129582 for ; Wed, 27 Mar 2019 11:04:34 -0400 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0b-001b2d01.pphosted.com with ESMTP id 2rgavy9rc8-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 27 Mar 2019 11:04:32 -0400 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 27 Mar 2019 15:04:04 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x2RF433w22806708 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 27 Mar 2019 15:04:03 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1C9EBB2064; Wed, 27 Mar 2019 15:04:03 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id F3AA1B206B; Wed, 27 Mar 2019 15:04:02 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 27 Mar 2019 15:04:02 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id A7D1516C60F9; Wed, 27 Mar 2019 08:04:02 -0700 (PDT) Date: Wed, 27 Mar 2019 08:04:02 -0700 From: "Paul E. McKenney" To: NeilBrown Cc: Herbert Xu , Thomas Graf , netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash bucket. Reply-To: paulmck@linux.ibm.com References: <155349021177.1111.15681654355431465791.stgit@noble.brown> <155349033961.1111.18247269615646768227.stgit@noble.brown> <20190326050320.gwk3tgtqwl5csivt@gondor.apana.org.au> <874l7p463d.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <874l7p463d.fsf@notabene.neil.brown.name> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19032715-0060-0000-0000-00000322CEA4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010823; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000282; SDB=6.01180448; UDB=6.00617762; IPR=6.00961165; MB=3.00026180; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-27 15:04:05 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19032715-0061-0000-0000-000048BEA4DF Message-Id: <20190327150402.GX4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-27_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-1903270106 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Mar 27, 2019 at 09:35:18AM +1100, NeilBrown wrote: > On Tue, Mar 26 2019, Herbert Xu wrote: > > > On Mon, Mar 25, 2019 at 04:05:39PM +1100, NeilBrown wrote: > >> > >> + * Sometimes we unlock a bucket by writing a new pointer there. In that > >> + * case we don't need to unlock, but we do need to reset state such as > >> + * local_bh. For that we have rht_unlocked(). This doesn't include > >> + * the memory barrier that bit_spin_unlock() provides, but rcu_assign_pointer() > >> + * will have provided that. > > > > Hmm, are you sure that's enough? IIRC rcu_assign_pointer only > > provides a write barrier compared to the more complete (but one-way) > > barrier that a spin-lock provides. > > > > The bit_spin_unlock(), which I am avoiding as unnecessary, would have > provided release semantics. > i.e. any write by this CPU that happened before the releasing write > will be visible to other CPUs before (or when) they see the result of > the releasing write. > This is (as I understand it) exactly that rcu_assign_pointer() promises > - even before acquire semantics were added as Paul just reported. > > So yes, I am sure (surer now that I've walked through it carefully). But why not construct a litmus test and apply tools/memory-model? ;-) Thanx, Paul