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 B3663C34026 for ; Tue, 18 Feb 2020 11:47:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 87E6C207FD for ; Tue, 18 Feb 2020 11:47:03 +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="Wb1uzUTH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726556AbgBRLrD (ORCPT ); Tue, 18 Feb 2020 06:47:03 -0500 Received: from merlin.infradead.org ([205.233.59.134]:41880 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726486AbgBRLrD (ORCPT ); Tue, 18 Feb 2020 06:47:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=UxjyAte6Pn7aIA1sKonsxJ1h6Ij2+oKQk3yC+HQWKYw=; b=Wb1uzUTHlM0O+2/DCKc77awEdC AP+b2dZyhAGrtdOPsxRJlpdm2XJ/mcOPG9fx/PcZquMGWMOQ3+oi5Tcin4GmToKywuMXnYDEKxYBg fDTXykpIMZdyieeiaMCT10LlSkBFa0l0TorVVd71EvkC7OHjst104lA8QOmCIJFJFHRB6xKEi4dTx o0RzZ8GlXUYh4nPaQyoVZuhRzO3Fiaf4tlAyOkm6RKq0/Ax1LCQ2N3NztkiVBcH/D3KPvvIuI66C4 PaBkzXYtYGm44sh6NdZt3aTosJNSb5FuWNpRweBKXtGRGGnezNApxOGv2yREiz2CpUI7XXAKYCrAv 1DsvWu8A==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j41Kl-0002iV-OF; Tue, 18 Feb 2020 11:46:35 +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 1AE00300446; Tue, 18 Feb 2020 12:44:40 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id F1CE4203E6A01; Tue, 18 Feb 2020 12:46:31 +0100 (CET) Date: Tue, 18 Feb 2020 12:46:31 +0100 From: Peter Zijlstra To: "Paul E. McKenney" 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: <20200218114631.GY14914@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200217230657.GA8985@paulmck-ThinkPad-P72> 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 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) > > 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) {