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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 D3EA2C4332F for ; Wed, 22 Sep 2021 11:27:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B87A360555 for ; Wed, 22 Sep 2021 11:27:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235655AbhIVL3H (ORCPT ); Wed, 22 Sep 2021 07:29:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42284 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235422AbhIVL3F (ORCPT ); Wed, 22 Sep 2021 07:29:05 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A5F3FC061574; Wed, 22 Sep 2021 04:27:35 -0700 (PDT) Date: Wed, 22 Sep 2021 13:27:31 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1632310053; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lZv1FAoayMtDLfu2V7KCrqzRbBVSYCgTHmUKvw7n0TI=; b=RpD/ipNJU/V9C8NM3rFlIfTfc8M6xHrRUWi49VqPb3sh6C71LywYxpfd4c/7X9bo/j12F8 zdfVJHeas6gDOPwHpvsH1kIk7Jg7lgUXm8xFFqjxVvX4CxkbyYryzABeG+SnTEMmaOJNaV K68O647TSPE3haj1p6pWz12nF1wdfDtzv1JG8d0dvs+Jna/oXnfsrhU2chW4v1f0D/Gd3z 6sAlUkZhZH8klmLYYyTEIM9KdrA7rFTfVqxiYqkCmNiuCWhB2oJrx1EA9dLaRplVMYUi9g t0HqJK0dkFWEDEJB9w99dGm+SZfXaRF1yWMWAPLeKW1RK3H9RDy7wpFAVmwhQA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1632310053; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lZv1FAoayMtDLfu2V7KCrqzRbBVSYCgTHmUKvw7n0TI=; b=vgdtmoLl6YC6cvTD52PgjGrxZM3dH+++Cx9uLI+f6wmGz0BblrypMUyshFohdUyDgLH21H nTbLXEYftIkFqwDw== From: Sebastian Andrzej Siewior To: Frederic Weisbecker Cc: Thomas Gleixner , Valentin Schneider , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org, linux-rt-users@vger.kernel.org, Catalin Marinas , Will Deacon , Ingo Molnar , Peter Zijlstra , Steven Rostedt , Daniel Bristot de Oliveira , "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , Davidlohr Bueso , Lai Jiangshan , Joel Fernandes , Anshuman Khandual , Vincenzo Frascino , Steven Price , Ard Biesheuvel , Boqun Feng , Mike Galbraith Subject: Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Message-ID: <20210922112731.dvauvxlhx5suc7qd@linutronix.de> References: <20210811201354.1976839-1-valentin.schneider@arm.com> <20210811201354.1976839-4-valentin.schneider@arm.com> <874kae6n3g.ffs@tglx> <87pmt163al.ffs@tglx> <20210921234518.GB100318@lothringen> <20210922063208.ltf7sdou4tr5yrnc@linutronix.de> <20210922111012.GA106513@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210922111012.GA106513@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-09-22 13:10:12 [+0200], Frederic Weisbecker wrote: > On Wed, Sep 22, 2021 at 08:32:08AM +0200, Sebastian Andrzej Siewior wrote: > > On 2021-09-22 01:45:18 [+0200], Frederic Weisbecker wrote: > > > > > > Also while at it, I'm asking again: traditionally softirqs could assume that > > > manipulating a local state was safe against !irq_count() code fiddling with > > > the same state on the same CPU. > > > > > > Now with preemptible softirqs, that assumption can be broken anytime. RCU was > > > fortunate enough to have a warning for that. But who knows how many issues like > > > this are lurking? > > > > If "local state" is modified then it is safe as long as it is modified > > within a local_bh_disable() section. And we are in this section while > > invoking a forced-threaded interrupt. The special part about RCU is > > that it is used in_irq() as part of core-code. > > But local_bh_disable() was deemed for protecting from interrupting softirqs, > not the other way around (softirqs being preempted by other tasks). The latter > semantic is new and nobody had that in mind until softirqs have been made > preemptible. > > For example: > > CPU 0 > ----------------------------------------------- > SOFTIRQ RANDOM TASK > ------ ----------- > int *X = &per_cpu(CPUX, 0) int *X = &per_cpu(CPUX, 0) > int A, B; WRITE_ONCE(*X, 0); > WRITE_ONCE(*X, 1); > A = READ_ONCE(*X); > B = READ_ONCE(*X); > > > We used to have the guarantee that A == B. That's not true anymore. Now > some new explicit local_bh_disable() should be carefully placed on RANDOM_TASK > where it wasn't necessary before. RCU is not that special in this regard. The part with rcutree.use_softirq=0 on RT does not make it any better, right? So you rely on some implicit behaviour which breaks with RT such as: CPU 0 ----------------------------------------------- RANDOM TASK-A RANDOM TASK-B ------ ----------- int *X = &per_cpu(CPUX, 0) int *X = &per_cpu(CPUX, 0) int A, B; spin_lock(&D); spin_lock(&C); WRITE_ONCE(*X, 0); A = READ_ONCE(*X); WRITE_ONCE(*X, 1); B = READ_ONCE(*X); while spinlock C and D are just random locks not related to CPUX but it just happens that they are held at that time. So for !RT you guarantee that A == B while it is not the case on RT. Sebastian