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=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 F0E4EC34044 for ; Tue, 18 Feb 2020 16:34:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C515222B48 for ; Tue, 18 Feb 2020 16:34:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582043646; bh=k6r7kW/VRYWSj/+4iFogeBzp0brmIrHpaYxLxUwi880=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=j8hKixl3XE1hq4dKotQY0uO3yDocj5Do9UKDK/nK+7cyJx6Jl/WwE7OaAWYnftlPi oyBbgZlIjj05FikQ7UhR2lmCqB25zEUlHJNZnwrzYx9mDso4YSTUhifSdWHXznajE0 jeoLmz8p0tRBB8NU1+bpHJZEw18El+59lVJ4DrLI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726567AbgBRQeG (ORCPT ); Tue, 18 Feb 2020 11:34:06 -0500 Received: from mail.kernel.org ([198.145.29.99]:57540 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726411AbgBRQeG (ORCPT ); Tue, 18 Feb 2020 11:34:06 -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 8238A2067D; Tue, 18 Feb 2020 16:34:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582043645; bh=k6r7kW/VRYWSj/+4iFogeBzp0brmIrHpaYxLxUwi880=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=nSQoAZS9Bz6zOOsa52dFQooE6a4TYCI7Eh5tRmUKLuk9+aPKKgUxvFhCnaJZssR5t nbdi5lupCJ7tad1IcRPM7HK7VSAVsbgEV7SQqJ3WoXVA9lVEZPZa03sXk5CTnazrlA Jwug8B7g3LnL4h5ChKEoh+/Ve6ifcoj5q9vt90zs= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 57E883520365; Tue, 18 Feb 2020 08:34:05 -0800 (PST) Date: Tue, 18 Feb 2020 08:34:05 -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: <20200218163405.GF2935@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> <20200217183220.GS2935@paulmck-ThinkPad-P72> <20200218114334.GX14914@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218114334.GX14914@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Feb 18, 2020 at 12:43:34PM +0100, Peter Zijlstra wrote: > On Mon, Feb 17, 2020 at 10:32:20AM -0800, Paul E. McKenney wrote: > > 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! > > Well, I didn't get further than the Changelog fails to describe an > actual problem and it looks like compare-against-a-constant. > > (worse, it masks off everything but the 2 lowest bits, so even if there > was a problem with load-tearing, it still wouldn't matter) There is still the possibility of load fusing. And the possibility of defending against possible future changes as well as the current snapshot of the code base. > I'm not going to argue with you if you want to use READ_ONCE() vs > data_race() and a comment to denote false-positive KCSAN warnings, but I > do feel somewhat strongly that the Changelog should describe the actual > problem -- if there is one -- or just flat out state that this is to > make KCSAN shut up but the code is fine. The problem is that "the code is fine" is highly subjective and varies over time. :-/ But in this case there was a real problem, just that I got confused when analyzing. > That is; every KCSAN report should be analysed, right? All I'm asking is > for that analysis to end up in the Changelog. Before responding further, I have to ask... Are you intending your "every KCSAN report should be analyzed" to apply globally or just when someone creates a patch based on such a report? In any case, you have acked this patch's successor (thank you very much!), so on this specific patch (or more accurately, its successor) I presume that we are all good. 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 > > > >