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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 CE6AEC33CAF for ; Thu, 23 Jan 2020 09:39:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A449B21734 for ; Thu, 23 Jan 2020 09:39:12 +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="G5E6yCVW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726761AbgAWJjL (ORCPT ); Thu, 23 Jan 2020 04:39:11 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:53970 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726026AbgAWJjL (ORCPT ); Thu, 23 Jan 2020 04:39:11 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=md0ZAk3FQElHi7XKV64Hv8xf0dK+/8lwA6XrQZmfofk=; b=G5E6yCVWS3GRTr6UKlwVnV6af qCnyJhNhT8PUON1JNDidqpblUj1Fn++0bPz8/SfRDfSe3Xp9WrQN673jD4O14VPhL6cniwYnjbKam CPpH2oEjhHv1IKSlmC8zSvG6zjzJTHSW/85N/xc6v2gPZVj6ijzGEFf3daci6NPYXU5z4g4KPFhhW nVunhbWNVSPwdTyd56xWu8x4MfbHYCbZb+VvUqOhpdeQoxxDvvcVXq/zTItkK2PYKH/SN40UVXZmc y9kCP5QA+tFinoR7Q/LB7mlyzTGi+9VuAdbHbOt+UcVlnpiIVec3H/mtamQGiWeHPKK/RCSNDek8B LXGatyHHg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1iuYx8-0003iU-Tx; Thu, 23 Jan 2020 09:39:08 +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 29A463042BC; Thu, 23 Jan 2020 10:37:25 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 264782B713024; Thu, 23 Jan 2020 10:39:05 +0100 (CET) Date: Thu, 23 Jan 2020 10:39:05 +0100 From: Peter Zijlstra To: Qian Cai Cc: Marco Elver , Will Deacon , mingo@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next Message-ID: <20200123093905.GU14914@hirez.programming.kicks-ass.net> References: <20200122165938.GA16974@willie-the-truck> <20200122223851.GA45602@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 22, 2020 at 06:54:43PM -0500, Qian Cai wrote: > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > index 1f7734949ac8..832e87966dcf 100644 > --- a/kernel/locking/osq_lock.c > +++ b/kernel/locking/osq_lock.c > @@ -75,7 +75,7 @@ osq_wait_next(struct optimistic_spin_queue *lock, > * wait for either @lock to point to us, through its Step-B, or > * wait for a new @node->next from its Step-C. > */ > - if (node->next) { > + if (READ_ONCE(node->next)) { > next = xchg(&node->next, NULL); > if (next) > break; This could possibly trigger the warning, but is a false positive. The above doesn't fix anything in that even if that load is shattered the code will function correctly -- it checks for any !0 value, any byte composite that is !0 is sufficient. This is in fact something KCSAN compiler infrastructure could deduce.