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=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 17A49C433E4 for ; Fri, 17 Jul 2020 17:47:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E190420717 for ; Fri, 17 Jul 2020 17:47:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="U1Kt4wEo" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727060AbgGQRr4 (ORCPT ); Fri, 17 Jul 2020 13:47:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46642 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726232AbgGQRrz (ORCPT ); Fri, 17 Jul 2020 13:47:55 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EBDD6C0619D2; Fri, 17 Jul 2020 10:47:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=5bNn1iv9D2whBZXnI8V5/oS3EG1YDPh+syl8DNPsUhU=; b=U1Kt4wEojI31oe5jA4aI9n41ye RZezlbwJ3X5butEOMLladO4jta0f0vUwJqYU8ihCRZ4yFReYwo0/Vjrmyq8kRDeBHPXt7UfwAyHyy pWahWEldkFExAGO4V2T1OYGcSJ2FFPQ/3j4copIVUZl+bizT33QXFj5hboCKts4RC/KrpM9Q7MmNw okSWSQXnJpfB2uAt3vOcAP/djy3NIRDRn8odyEKYx4QzyHGFcS2vEXMZl7Qf611ePcwtNPZP+1fTI vGzba+AjWykxIei1LwB1yuSoS6pz9LTNanBUxQoBw3No3wPpKcQ/8ba/Gi+EZCOBB/o80CYg3EBQV puExjdtQ==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jwUSc-00036X-Lw; Fri, 17 Jul 2020 17:47:50 +0000 Date: Fri, 17 Jul 2020 18:47:50 +0100 From: Matthew Wilcox To: Eric Biggers Cc: 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 , "Darrick J . Wong" , 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: <20200717174750.GQ12769@casper.infradead.org> References: <20200717044427.68747-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200717044427.68747-1-ebiggers@kernel.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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!