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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 17E76C433E3 for ; Fri, 12 Jun 2020 14:48:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EEF8D206A4 for ; Fri, 12 Jun 2020 14:48:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726668AbgFLOsA (ORCPT ); Fri, 12 Jun 2020 10:48:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58778 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726581AbgFLOrx (ORCPT ); Fri, 12 Jun 2020 10:47:53 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0C31BC03E96F; Fri, 12 Jun 2020 07:47:53 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jjky9-0005Nb-33; Fri, 12 Jun 2020 16:47:45 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 71061100F5A; Fri, 12 Jun 2020 16:47:44 +0200 (CEST) From: Thomas Gleixner To: Frederic Weisbecker Cc: "Paul E. McKenney" , LKML , rcu@vger.kernel.org, Andrew Lutomirski , X86 ML , Steven Rostedt , Joel Fernandes , Mathieu Desnoyers , Will Deacon , Peter Zijlstra Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task In-Reply-To: <20200612142621.GA8009@lenoir> References: <20200611235305.GA32342@paulmck-ThinkPad-P72> <871rmkzcc8.fsf@nanos.tec.linutronix.de> <87wo4cxubv.fsf@nanos.tec.linutronix.de> <20200612142621.GA8009@lenoir> Date: Fri, 12 Jun 2020 16:47:44 +0200 Message-ID: <87tuzgxrvz.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Frederic Weisbecker writes: > On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote: >> The idea of conditionally calling into rcu_irq_enter() only when RCU is >> not watching turned out to be not completely thought through. >> >> Paul noticed occasional premature end of grace periods in RCU torture >> testing. Bisection led to the commit which made the invocation of >> rcu_irq_enter() conditional on !rcu_is_watching(). >> >> It turned out that this conditional breaks RCU assumptions about the idle >> task when the scheduler tick happens to be a nested interrupt. Nested >> interrupts can happen when the first interrupt invokes softirq processing >> on return which enables interrupts. If that nested tick interrupt does not >> invoke rcu_irq_enter() then the nest accounting in RCU claims that this is >> the first interrupt which might mark a quiescient state and end grace >> periods prematurely. >> >> Change the condition from !rcu_is_watching() to is_idle_task(current) which >> enforces that interrupts in the idle task unconditionally invoke >> rcu_irq_enter() independent of the RCU state. >> >> This is also correct vs. user mode entries in NOHZ full scenarios because >> user mode entries bring RCU out of EQS and force the RCU irq nesting state >> accounting to nested. As only the first interrupt can enter from user mode >> a nested tick interrupt will enter from kernel mode and as the nesting >> state accounting is forced to nesting it will not do anything stupid even >> if rcu_irq_enter() has not been invoked. >> >> Fixes: 3eeec3858488 ("x86/entry: Provide idtentry_entry/exit_cond_rcu()") >> Reported-by: "Paul E. McKenney" >> Signed-off-by: Thomas Gleixner > > Acked-by: Frederic Weisbecker > > So, in the end the call to rcu_irq_enter() in irq_enter() is going to > be useless in x86, right? x86 is not calling irq_enter() anymore. It's using irq_enter_rcu().