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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 ADB71C433E0 for ; Fri, 12 Jun 2020 15:32:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D48420835 for ; Fri, 12 Jun 2020 15:32:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591975948; bh=1bzFs/3D9de07O+VVT9Y1Exb8KtBe+TWOs5uYgeoTAs=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=U/9Yu3HF1oYO9fpYsScnUs6TOh8MNvpjuqz7hWYADr0Uhf3Kg6kJ65mScyFpOoTv3 LX/8oyk3gbLnGB5psOW0rg30RqfxKx5F7pS1EAwrOlUvkb1ajINbrSxjfBwyVO5fFa rtQvt3KgEt/LbWifRCf/DIFdyKCag9oGkq6nJz94= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726514AbgFLPc1 (ORCPT ); Fri, 12 Jun 2020 11:32:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:50606 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726341AbgFLPc1 (ORCPT ); Fri, 12 Jun 2020 11:32:27 -0400 Received: from mail-wr1-f46.google.com (mail-wr1-f46.google.com [209.85.221.46]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2DCDB20897 for ; Fri, 12 Jun 2020 15:32:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591975946; bh=1bzFs/3D9de07O+VVT9Y1Exb8KtBe+TWOs5uYgeoTAs=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=t/BJGapHgyyVp0rAEapRuPoZCWwU9+VAHlCvOKkUD1WLOOAqzV1TofaFn4eixuiTb wW3NXMUj+GKBzxbCdX4YvJLIsvoeSS+QqOnELPRBiSDsEFKZYoNTiIMDVR4Lb7YbEc HnFDylLPtVaKl7IFOYeDkFzvbeacoITpPU0JARPQ= Received: by mail-wr1-f46.google.com with SMTP id l10so10149265wrr.10 for ; Fri, 12 Jun 2020 08:32:26 -0700 (PDT) X-Gm-Message-State: AOAM533dv1If+7XXGud5apYRVyZwcnS1O2RemzHejlFH3D7sNZaj5z/a aSyBIlBzBgjUg5SlvJRa4nYLX2nNtTzDClwLRq8WSA== X-Google-Smtp-Source: ABdhPJzQB9LssgQGmvqHFwk9xZh2VrVhNFjOs6eTIw1uvdewBQcpNsBpLgMhosfLlEhD0545J0Q0fO+qWejcN9qscNA= X-Received: by 2002:a5d:49c5:: with SMTP id t5mr15864784wrs.18.1591975944724; Fri, 12 Jun 2020 08:32:24 -0700 (PDT) MIME-Version: 1.0 References: <20200611235305.GA32342@paulmck-ThinkPad-P72> <871rmkzcc8.fsf@nanos.tec.linutronix.de> <87wo4cxubv.fsf@nanos.tec.linutronix.de> In-Reply-To: <87wo4cxubv.fsf@nanos.tec.linutronix.de> From: Andy Lutomirski Date: Fri, 12 Jun 2020 08:32:13 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task To: Thomas Gleixner Cc: "Paul E. McKenney" , LKML , rcu@vger.kernel.org, Andrew Lutomirski , X86 ML , Frederic Weisbecker , Steven Rostedt , Joel Fernandes , Mathieu Desnoyers , Will Deacon , Peter Zijlstra Content-Type: text/plain; charset="UTF-8" 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 6:55 AM 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. Acked-by: Andy Lutomirski