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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 6D586C34022 for ; Mon, 17 Feb 2020 18:32:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4A52A24693 for ; Mon, 17 Feb 2020 18:32:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581964343; bh=rvWTyaYeifjPqyUfCKPJ7kkPbaQPPWSrX34cC7WHYzA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=wjO8T383WJ33ZQQppSRFWsjdly7f7E5zPAWU8F3uPcqkzqCUjV672+FlIOOU0loym jFmNmHzc5O0raR7J5tTSSMPh/o6QC+mn04SHq8nmuQCTCLN8HrboU5vCVeZp7RJpiw dRfO5vQtcqGkW3SvagrsXKDYT172v85beVjSCjoI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728676AbgBQScW (ORCPT ); Mon, 17 Feb 2020 13:32:22 -0500 Received: from mail.kernel.org ([198.145.29.99]:47706 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726707AbgBQScV (ORCPT ); Mon, 17 Feb 2020 13:32:21 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 0E25724689; Mon, 17 Feb 2020 18:32:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581964341; bh=rvWTyaYeifjPqyUfCKPJ7kkPbaQPPWSrX34cC7WHYzA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=dAROZkYWdkfyGo9BVUpJRwP4FNQYeRsiSbak30ehQKGHbQu9GMuWxYsTZMLZsNWlL PVkVBLV4PEA8jiiNczz5BY+PsNIDGdey+H/mx05ZCegrUkgx67Qqi85uF7Iqx1JqG8 aI9r9c3Z6PvNOmo6X4DLh5Lm5eHwXktW+DFnvDVQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id CF02D352273C; Mon, 17 Feb 2020 10:32:20 -0800 (PST) Date: Mon, 17 Feb 2020 10:32:20 -0800 From: "Paul E. McKenney" To: Peter Zijlstra 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: <20200217183220.GS2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200215002907.GA15895@paulmck-ThinkPad-P72> <20200215002932.15976-4-paulmck@kernel.org> <20200217124507.GT14914@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200217124507.GT14914@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 17, 2020 at 01:45:07PM +0100, Peter Zijlstra wrote: > 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. Now that you mention it, the lock is held at that point, isn't it? So if that READ_ONCE() actually does anything, there is a bug somewhere else. Good catch, I will drop this patch, thank you! Thanx, Paul > > 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 > >