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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 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 291F4C352A3 for ; Sat, 15 Feb 2020 03:47:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 027AD20873 for ; Sat, 15 Feb 2020 03:47:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727883AbgBODrs (ORCPT ); Fri, 14 Feb 2020 22:47:48 -0500 Received: from mail.kernel.org ([198.145.29.99]:51290 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727642AbgBODrr (ORCPT ); Fri, 14 Feb 2020 22:47:47 -0500 Received: from oasis.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (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 79C602083B; Sat, 15 Feb 2020 03:47:45 +0000 (UTC) Date: Fri, 14 Feb 2020 22:47:43 -0500 From: Steven Rostedt 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, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Linus Torvalds Subject: Re: [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store Message-ID: <20200214224743.280772a7@oasis.local.home> In-Reply-To: <20200214235607.13749-6-paulmck@kernel.org> References: <20200214235536.GA13364@paulmck-ThinkPad-P72> <20200214235607.13749-6-paulmck@kernel.org> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 14 Feb 2020 15:55:43 -0800 paulmck@kernel.org wrote: > From: "Paul E. McKenney" > > The rcu_node structure's ->exp_seq_rq field is read locklessly, so > this commit adds the WRITE_ONCE() to a load in order to provide proper > documentation and READ_ONCE()/WRITE_ONCE() pairing. > > This data race was reported by KCSAN. Not appropriate for backporting > due to failure being unlikely. > > Signed-off-by: Paul E. McKenney > --- > kernel/rcu/tree_exp.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index d7e0484..85b009e 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s) > sync_exp_work_done(s)); > return true; > } > - rnp->exp_seq_rq = s; /* Followers can wait on us. */ > + WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */ Didn't Linus say this is basically bogus? Perhaps just using it as documenting that it's read locklessly, but is it really needed? -- Steve > spin_unlock(&rnp->exp_lock); > trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level, > rnp->grplo, rnp->grphi, TPS("nxtlvl"));