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=-3.5 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 3D830C433E6 for ; Fri, 28 Aug 2020 15:30:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1470920825 for ; Fri, 28 Aug 2020 15:30:15 +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="annfokD/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726197AbgH1PaO (ORCPT ); Fri, 28 Aug 2020 11:30:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37124 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725969AbgH1PaN (ORCPT ); Fri, 28 Aug 2020 11:30:13 -0400 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C9766C061264; Fri, 28 Aug 2020 08:30:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=0zFcRLKj8ImMCcxbtxiQYDljXZ++TUrgC02+xq5Z1lM=; b=annfokD/Of/+tc7NAID/pa8TGk HhUHWxQbCb9vUFIi0nEEwEz3Zpsg1jhgSOIdw9b+SQ86JNA3dVRgLPJ2YXxwh6rvR+ZBQROXsvoSj J5X/+AAGPuKQAI+kJcdQbGXdXzvb2+aUbm7Z+vZNs0xlYXIbGp70W00SeGZ0XtyrwzGFOCwK+d+Sf t0NhCiOIYSU/zxqTae5rbrVt7hU/tIKpdJz0/1wPlaGFz4/ph21pUMfNV5/I9ps699SMPUMC32z/k wdLy7d/eQR8kNV4Cyk1NQv7VfkFmOvqcfbuN3DdNdjsDIyFZobBspz8cCfqKzlmDB35gRg7lFvVQa xVR3Sa1A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kBgK5-0002ff-2A; Fri, 28 Aug 2020 15:29:49 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 7C83D3003E5; Fri, 28 Aug 2020 17:29:46 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 659E02C5FEF2B; Fri, 28 Aug 2020 17:29:46 +0200 (CEST) Date: Fri, 28 Aug 2020 17:29:46 +0200 From: peterz@infradead.org To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, mhiramat@kernel.org, Eddy_Wu@trendmicro.com, x86@kernel.org, davem@davemloft.net, rostedt@goodmis.org, naveen.n.rao@linux.ibm.com, anil.s.keshavamurthy@intel.com, linux-arch@vger.kernel.org, cameron@moodycamel.com, will@kernel.org, paulmck@kernel.org Subject: Re: [RFC][PATCH 6/7] freelist: Lock less freelist Message-ID: <20200828152946.GG1362448@hirez.programming.kicks-ass.net> References: <20200827161237.889877377@infradead.org> <20200827161754.535381269@infradead.org> <20200828144650.GF28468@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200828144650.GF28468@redhat.com> Sender: linux-arch-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Fri, Aug 28, 2020 at 04:46:52PM +0200, Oleg Nesterov wrote: > On 08/27, Peter Zijlstra wrote: > > > > 1 file changed, 129 insertions(+) > > 129 lines! And I spent more than 2 hours trying to understand these > 129 lines ;) looks correct... Yes, even though it already has a bunch of comments, I do feel we can maybe improve on that a little. For now I went for a 1:1 transliteration of the blog post though. > > + /* > > + * Yay, got the node. This means it was on the list, > > + * which means should-be-on-freelist must be false no > > + * matter the refcount (because nobody else knows it's > > + * been taken off yet, it can't have been put back on). > > + */ > > + WARN_ON_ONCE(atomic_read(&head->refs) & REFS_ON_FREELIST); > > + > > + /* > > + * Decrease refcount twice, once for our ref, and once > > + * for the list's ref. > > + */ > > + atomic_fetch_add(-2, &head->refs); > > Do we the barriers implied by _fetch_? Why can't atomic_sub(2, refs) work? I think we can, the original has std::memory_order_relaxed here. So I should've used atomic_fetch_add_relaxed() but since we don't use the return value, atomic_sub() would work just fine too. > > + /* > > + * OK, the head must have changed on us, but we still need to decrement > > + * the refcount we increased. > > + */ > > + refs = atomic_fetch_add(-1, &prev->refs); > > Cosmetic, but why not atomic_fetch_dec() ? The original had that, I didn't want to risk more bugs by 'improving' things. But yes, that can definitely become dec().