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=-4.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY autolearn=no 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 907B2C433DF for ; Fri, 17 Jul 2020 21:06:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 69AC120704 for ; Fri, 17 Jul 2020 21:06:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="LGojB183" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726788AbgGQVGF (ORCPT ); Fri, 17 Jul 2020 17:06:05 -0400 Received: from userp2130.oracle.com ([156.151.31.86]:47368 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726399AbgGQVGF (ORCPT ); Fri, 17 Jul 2020 17:06:05 -0400 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 06HL2NZS085868; Fri, 17 Jul 2020 21:05:42 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to; s=corp-2020-01-29; bh=f1RfLG9rSTeLiGLq95U2E9ilsHk1yVv7O8p3UH8YgSA=; b=LGojB183+7b5/wM3AQ/dmpbv8OPRsfv5S17Uu1/UXRwD6eRsEG2/zc9g4g7FDkl1rrEj pw67rVkXq8TNqIPqIF8DUDpaLDbWZnXAbCOjmuuyBD4uO6UmGp+1vIJnNL2yHKpX1bPZ l1aeCbmEeTAaDvDX9oXWUbOCggKoBpog6f+8Rh4BwjN4XZAbBBKc3ZLvUUIa/m94ZyoP AvwLsVc4SU5ZY3AvDHVvue9zfTUTzTGmIAGO+45BA3GIkC/JitG4hTUllko8OHjDfcAb Gs6PYZT+K3IjhfEWxIMr3yx6pEK0vX0Ku2uWHFJ6ROHd4IYCB8f5DtP8LeNlMT1G8qkF Ow== Received: from aserp3030.oracle.com (aserp3030.oracle.com [141.146.126.71]) by userp2130.oracle.com with ESMTP id 3274ursh64-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Fri, 17 Jul 2020 21:05:42 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 06HL3qCo002912; Fri, 17 Jul 2020 21:05:42 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3030.oracle.com with ESMTP id 32bj7gunus-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 17 Jul 2020 21:05:41 +0000 Received: from abhmp0010.oracle.com (abhmp0010.oracle.com [141.146.116.16]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 06HL5dnd030869; Fri, 17 Jul 2020 21:05:39 GMT Received: from localhost (/10.159.159.76) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 17 Jul 2020 14:05:39 -0700 Date: Fri, 17 Jul 2020 14:05:36 -0700 From: "Darrick J. Wong" To: Matthew Wilcox Cc: Eric Biggers , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Paul E . McKenney" , linux-fsdevel@vger.kernel.org, Akira Yokosawa , Alan Stern , Andrea Parri , Boqun Feng , Daniel Lustig , Dave Chinner , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , Peter Zijlstra , Will Deacon Subject: Re: [PATCH] tools/memory-model: document the "one-time init" pattern Message-ID: <20200717210536.GQ3151642@magnolia> References: <20200717044427.68747-1-ebiggers@kernel.org> <20200717174750.GQ12769@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200717174750.GQ12769@casper.infradead.org> X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9685 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=100 malwarescore=0 phishscore=0 adultscore=0 suspectscore=0 mlxscore=0 bulkscore=0 mlxlogscore=479 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2007170142 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9685 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=100 lowpriorityscore=0 impostorscore=0 suspectscore=0 phishscore=0 spamscore=0 mlxlogscore=496 malwarescore=0 mlxscore=0 priorityscore=1501 adultscore=0 bulkscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2007170142 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote: > On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote: > > +If that doesn't apply, you'll have to implement one-time init yourself. > > + > > +The simplest implementation just uses a mutex and an 'inited' flag. > > +This implementation should be used where feasible: > > I think some syntactic sugar should make it feasible for normal people > to implement the most efficient version of this just like they use locks. > > > +For the single-pointer case, a further optimized implementation > > +eliminates the mutex and instead uses compare-and-exchange: > > + > > + static struct foo *foo; > > + > > + int init_foo_if_needed(void) > > + { > > + struct foo *p; > > + > > + /* pairs with successful cmpxchg_release() below */ > > + if (smp_load_acquire(&foo)) > > + return 0; > > + > > + p = alloc_foo(); > > + if (!p) > > + return -ENOMEM; > > + > > + /* on success, pairs with smp_load_acquire() above and below */ > > + if (cmpxchg_release(&foo, NULL, p) != NULL) { > > Why do we have cmpxchg_release() anyway? Under what circumstances is > cmpxchg() useful _without_ having release semantics? > > > + free_foo(p); > > + /* pairs with successful cmpxchg_release() above */ > > + smp_load_acquire(&foo); > > + } > > + return 0; > > + } > > How about something like this ... > > once.h: > > static struct init_once_pointer { > void *p; > }; > > static inline void *once_get(struct init_once_pointer *oncep) > { ... } > > static inline bool once_store(struct init_once_pointer *oncep, void *p) > { ... } > > --- foo.c --- > > struct foo *get_foo(gfp_t gfp) > { > static struct init_once_pointer my_foo; > struct foo *foop; > > foop = once_get(&my_foo); > if (foop) > return foop; > > foop = alloc_foo(gfp); > if (!once_store(&my_foo, foop)) { > free_foo(foop); > foop = once_get(&my_foo); > } > > return foop; > } > > Any kernel programmer should be able to handle that pattern. And no mutex! I like it... :) --D