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=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 1FAD9C35254 for ; Mon, 17 Feb 2020 12:39:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EC80620725 for ; Mon, 17 Feb 2020 12:39:27 +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="i+f+hHeC" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728054AbgBQMj1 (ORCPT ); Mon, 17 Feb 2020 07:39:27 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:56366 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728025AbgBQMj1 (ORCPT ); Mon, 17 Feb 2020 07:39:27 -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; bh=IIpOmEB3A0357qhgGhhncaosuY31N7yX0gm83o8Xgi4=; b=i+f+hHeCnXrUHPRGfcrQvNIk6q daH4/g6Y7MdvLEm+CalG5XsK00TVtZ+i2Md3cJhlUA/utM9iP9peYR3tU3hJMIz+0vW/G29DLSzGN Zc1Ha97usEs/j3yGWbcINJlyDmVPFXDZY+NW+VyNOoKOkAqE2AuvRrglKkBN4b8ZIYX/2VVJzljGH vimGod0LaEL263CM0nFSLOlXT4JLJqk86mMpqEPVskiQRpz4naihIseIvPFhCrvybrWuA7X2483Bq CNc0qWeUcgWeBkDD5C5y2wuikQPDkgG/0ZyUSqMcB/QuzrxuRRafSb+tdB6/eM+gnJsDyaf1q4oXg 0DTNeHsA==; 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 1j3ffr-0004F8-6C; Mon, 17 Feb 2020 12:38:55 +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 ACF0D300EBB; Mon, 17 Feb 2020 13:37:00 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id EB64D29E09C26; Mon, 17 Feb 2020 13:38:51 +0100 (CET) Date: Mon, 17 Feb 2020 13:38:51 +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 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head Message-ID: <20200217123851.GR14914@hirez.programming.kicks-ass.net> References: <20200215002446.GA15663@paulmck-ThinkPad-P72> <20200215002520.15746-1-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200215002520.15746-1-paulmck@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Feb 14, 2020 at 04:25:18PM -0800, paulmck@kernel.org wrote: > From: "Paul E. McKenney" > > The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly > by rcu_tasks_kthread() when waiting for work to do. This commit therefore > applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the > single potential store outside of rcu_tasks_kthread. > > This data race was reported by KCSAN. Not appropriate for backporting > due to failure being unlikely. What failure is possible here? AFAICT this is (again) one of them load-complare-against-constant-discard patterns that are impossible to mess up. > Signed-off-by: Paul E. McKenney > --- > kernel/rcu/update.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > index 6c4b862..a27df76 100644 > --- a/kernel/rcu/update.c > +++ b/kernel/rcu/update.c > @@ -528,7 +528,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func) > rhp->func = func; > raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); > needwake = !rcu_tasks_cbs_head; > - *rcu_tasks_cbs_tail = rhp; > + WRITE_ONCE(*rcu_tasks_cbs_tail, rhp); > rcu_tasks_cbs_tail = &rhp->next; > raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); > /* We can't create the thread unless interrupts are enabled. */ > @@ -658,7 +658,7 @@ static int __noreturn rcu_tasks_kthread(void *arg) > /* If there were none, wait a bit and start over. */ > if (!list) { > wait_event_interruptible(rcu_tasks_cbs_wq, > - rcu_tasks_cbs_head); > + READ_ONCE(rcu_tasks_cbs_head)); > if (!rcu_tasks_cbs_head) { > WARN_ON(signal_pending(current)); > schedule_timeout_interruptible(HZ/10); > -- > 2.9.5 >