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 BF0EBC3404B for ; Tue, 18 Feb 2020 21:48:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 98F6822B48 for ; Tue, 18 Feb 2020 21:48:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582062500; bh=WiwCNvyJt8cbbiANZODpyVDUSfy10IYL4YvpxXMSn94=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=b83cKOnhWaDEWLXtfbHKPNp8j41HpvAnMv8X+lAX0qsCssxNozBkcSr4zR8VUeNbW nNwrOb3ff4t0aNMYtoAGSkow1HbKYEqCWFet6qb389TcqYll3p/LZyRp702VBrUmVz YlHz/vb6v0pbMK6dyzbZII8fG5RX1gc2gq1Q7awI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727972AbgBRVsT (ORCPT ); Tue, 18 Feb 2020 16:48:19 -0500 Received: from mail.kernel.org ([198.145.29.99]:48752 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727868AbgBRVsT (ORCPT ); Tue, 18 Feb 2020 16:48:19 -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 99797206E2; Tue, 18 Feb 2020 21:48:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582062498; bh=WiwCNvyJt8cbbiANZODpyVDUSfy10IYL4YvpxXMSn94=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=fme/q6nIBndVPOE9S/+MT4Im1evsayUsfqULw+kjhN65iDdWtejOcHK+PIydugRKa GsWcJNqqWNm0cBzwFSa3xh267On4LF1y04UZOeUCMna3rT7bJAg3zaYq/xXNGqgWQK kjxxM+IfH9b4BaqXBlmFfannJryAHNbZ6ZGIyeKU= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 700173520856; Tue, 18 Feb 2020 13:48:18 -0800 (PST) Date: Tue, 18 Feb 2020 13:48:18 -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: <20200218214818.GM2935@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> <20200217230657.GA8985@paulmck-ThinkPad-P72> <20200218114631.GY14914@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218114631.GY14914@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 Tue, Feb 18, 2020 at 12:46:31PM +0100, Peter Zijlstra wrote: > On Mon, Feb 17, 2020 at 03:06:57PM -0800, Paul E. McKenney wrote: > > commit 52324a7b8a025f47a1a1a9fbd23ffe59fa764764 > > Author: Paul E. McKenney > > Date: Fri Jan 3 11:42:05 2020 -0800 > > > > srcu: Hold srcu_struct ->lock when updating ->srcu_gp_seq > > > > A read of the srcu_struct structure's ->srcu_gp_seq field should not > > need READ_ONCE() when that structure's ->lock is held. Except that this > > lock is not always held when updating this field. This commit therefore > > acquires the lock around updates and removes a now-unneeded READ_ONCE(). > > > > This data race was reported by KCSAN. > > > > Signed-off-by: Paul E. McKenney > > Acked-by: Peter Zijlstra (Intel) Applied, thank you! Thanx, Paul > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index 119a373..c19c1df 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -450,7 +450,7 @@ static void srcu_gp_start(struct srcu_struct *ssp) > > spin_unlock_rcu_node(sdp); /* Interrupts remain disabled. */ > > smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */ > > rcu_seq_start(&ssp->srcu_gp_seq); > > - state = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)); > > + state = rcu_seq_state(ssp->srcu_gp_seq); > > WARN_ON_ONCE(state != SRCU_STATE_SCAN1); > > } > > > > @@ -1130,7 +1130,9 @@ static void srcu_advance_state(struct srcu_struct *ssp) > > return; /* readers present, retry later. */ > > } > > srcu_flip(ssp); > > + spin_lock_irq_rcu_node(ssp); > > rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2); > > + spin_unlock_irq_rcu_node(ssp); > > } > > > > if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {