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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 4A48CC54FCC for ; Tue, 21 Apr 2020 16:25:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 214E9206E9 for ; Tue, 21 Apr 2020 16:25:11 +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="NO2cixJ2" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726878AbgDUQZK (ORCPT ); Tue, 21 Apr 2020 12:25:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38288 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1725930AbgDUQZI (ORCPT ); Tue, 21 Apr 2020 12:25:08 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7D9B3C061A10 for ; Tue, 21 Apr 2020 09:25:08 -0700 (PDT) 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=E+VZuP5UASvBlDVg5ti5uG0guhQC0Mhbi6U/Zkjtonc=; b=NO2cixJ2hIhwMNoJ1ZRM/R+AtG A56+vQ8wKWc0juCuxi51VCpVD9+dwqZ99vpf3VzITFoYd6NTCauBXrlBbYop/Fcq72SDYEDh9rLMe JoapZYEgV3j7wb+wtANIw6a8YivRbIKTKymEiaKvP+oAN7jzmoQmQFFfDYnHqCcRixCWm9oakn7Bf ImFJKUT1RzRvAgKo5ZWTmOLJewL+3oUQX009Gwwco47p9Ea9LpuGSeNevKZME1s5kkF3U3TEfa86O 5Kckis7VJtRNw1mO1TGWZK8eg2fYvxbCEYcNOkpheLUOazwj6YRP29v7Z9c5uq7aEKiq1HrDH02UG ndilG9Cg==; 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 1jQvhg-00022Z-Jq; Tue, 21 Apr 2020 16:24:56 +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 6980D306064; Tue, 21 Apr 2020 18:24:52 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 27ED12BAC8715; Tue, 21 Apr 2020 18:24:52 +0200 (CEST) Date: Tue, 21 Apr 2020 18:24:52 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Muchun Song , mingo@kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, joel@joelfernandes.org Subject: Re: [PATCH] sched/fair: Fix call walk_tg_tree_from() without hold rcu_lock Message-ID: <20200421162452.GV20730@hirez.programming.kicks-ass.net> References: <20200406121008.62903-1-songmuchun@bytedance.com> <20200421135258.GS20730@hirez.programming.kicks-ass.net> <20200421154312.GO17661@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200421154312.GO17661@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 21, 2020 at 08:43:12AM -0700, Paul E. McKenney wrote: > On Tue, Apr 21, 2020 at 03:52:58PM +0200, Peter Zijlstra wrote: > > On Mon, Apr 06, 2020 at 08:10:08PM +0800, Muchun Song wrote: > > > The walk_tg_tree_from() caller must hold rcu_lock, > > > > Not quite; with the RCU unification done 'recently' having preemption > > disabled is sufficient. AFAICT preemption is disabled. > > > > In fact; and I mentioned this to someone the other day, perhaps Joel; we > > can go and delete a whole bunch of rcu_read_lock() from the scheduler -- > > basically undo all the work we did after RCU was split many years ago. > > "If only I knew then what I know now..." > > Then again, I suspect that we all have ample opportunity to use that > particular old phrase. ;-) Quite so; I'm just fearing that rcu-lockdep annotation stuff. IIRC that doesn't (nor can it, in general) consider the implicit preempt-disable from locks and such for !PREEMPT builds.