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=-8.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 A80C5C761A6 for ; Mon, 17 Feb 2020 12:45:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8082B20725 for ; Mon, 17 Feb 2020 12:45:51 +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="mdlW35hN" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728687AbgBQMpu (ORCPT ); Mon, 17 Feb 2020 07:45:50 -0500 Received: from merlin.infradead.org ([205.233.59.134]:41862 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726397AbgBQMpt (ORCPT ); Mon, 17 Feb 2020 07:45:49 -0500 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=FelI1iG9O0CpNorqieVC1Sm3aQjwvDBCFxEQykU0JaM=; b=mdlW35hNjSoqGAmHrcq3Pl3S9I gVImasJMb74z/FdjbstMj7gbfhFrGVaProoFs/gOahV72Jb2VB6cl7cNKFwSy83qA3r5evh/U6+9r yX2UMSpvl/tzVViTfNxsExlzSlPmsmruCI/yKbvcpjHQ5hDTuy7GFkOrbovggmbRmkXrcNH+EwhXa 6EQI+klkY6CStdM8djQVB3rJIMisuqhan+0ufImKSKLQOg3zvN3EeHBh99pL3kkG1wzhc5HvJ7gLp x18ymWk6lWZIqRr4bU7lrVqKlB8pSwvs+VSUtPXilvp07j3QnS/JLz7HMhoFx6qy3AKVFKq9das12 hZhEhLUg==; 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 1j3flu-0003tS-Ag; Mon, 17 Feb 2020 12:45:10 +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 2B65E300EBB; Mon, 17 Feb 2020 13:43:15 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 538622B782E24; Mon, 17 Feb 2020 13:45:07 +0100 (CET) Date: Mon, 17 Feb 2020 13:45:07 +0100 From: Peter Zijlstra To: paulmck@kernel.org Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp_seq load Message-ID: <20200217124507.GT14914@hirez.programming.kicks-ass.net> References: <20200215002907.GA15895@paulmck-ThinkPad-P72> <20200215002932.15976-4-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200215002932.15976-4-paulmck@kernel.org> 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 Fri, Feb 14, 2020 at 04:29:32PM -0800, paulmck@kernel.org wrote: > From: "Paul E. McKenney" > > The load of the srcu_struct structure's ->srcu_gp_seq field in > srcu_funnel_gp_start() is lockless, so this commit adds the requisite > READ_ONCE(). > > This data race was reported by KCSAN. But is there in actual fact a data-race? AFAICT this code was just fine. > Signed-off-by: Paul E. McKenney > --- > kernel/rcu/srcutree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index 119a373..90ab475 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -678,7 +678,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > /* If grace period not already done and none in progress, start it. */ > if (!rcu_seq_done(&ssp->srcu_gp_seq, s) && > - rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) { > + rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_IDLE) { > WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)); > srcu_gp_start(ssp); > if (likely(srcu_init_done)) > -- > 2.9.5 >