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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 B27CBC433E0 for ; Fri, 12 Jun 2020 14:26:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 887E420878 for ; Fri, 12 Jun 2020 14:26:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591971987; bh=atjW46QNtnS8Y/KXlo++XpkVRSaZZCmd4cUh/5IL8Ng=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=azl8gzI2hNbcS5YS03NpfhmhcF7lcm8LMnoa5Y/T2VlxB7X6QqdrJ9wyBUaxI5F1E XcsyBVLwr8tKrc1TcTbNI6nnbnsEk/ZKwiE0OI/hKEvTAKuqIyB6ToMmWNgsJzVTKq iRjOtbgoLu3Qve8VMxXIsAzDYTLv4n21Ir1tg154= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726578AbgFLO00 (ORCPT ); Fri, 12 Jun 2020 10:26:26 -0400 Received: from mail.kernel.org ([198.145.29.99]:42496 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726255AbgFLO0Z (ORCPT ); Fri, 12 Jun 2020 10:26:25 -0400 Received: from localhost (lfbn-ncy-1-150-120.w83-194.abo.wanadoo.fr [83.194.232.120]) (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 625662074B; Fri, 12 Jun 2020 14:26:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591971984; bh=atjW46QNtnS8Y/KXlo++XpkVRSaZZCmd4cUh/5IL8Ng=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Jf0ZO9Byvv4MMEO2a22RpHiRwJQ3yGIMVrEHTRwfMEtBFVxwVh9XjL9hlleu9AYzq BiDf7R1PjOyYspYxchWS8g1P2kvh+wrsNqTK0H9X1ksNJkdm3LKUq0iOlpm+lSUOxC D5QXQmd9NKwv7cXnlA6WbDsp2Ql0FbFf3blGxv3g= Date: Fri, 12 Jun 2020 16:26:22 +0200 From: Frederic Weisbecker To: Thomas Gleixner 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 Message-ID: <20200612142621.GA8009@lenoir> References: <20200611235305.GA32342@paulmck-ThinkPad-P72> <871rmkzcc8.fsf@nanos.tec.linutronix.de> <87wo4cxubv.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wo4cxubv.fsf@nanos.tec.linutronix.de> 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 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?