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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,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 00441C76186 for ; Tue, 23 Jul 2019 17:08:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7ACB2184E for ; Tue, 23 Jul 2019 17:08:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390970AbfGWRIA (ORCPT ); Tue, 23 Jul 2019 13:08:00 -0400 Received: from foss.arm.com ([217.140.110.172]:57698 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726513AbfGWRIA (ORCPT ); Tue, 23 Jul 2019 13:08:00 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 64CD7337; Tue, 23 Jul 2019 10:07:59 -0700 (PDT) Received: from [10.1.196.105] (eglon.cambridge.arm.com [10.1.196.105]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0A6533F71A; Tue, 23 Jul 2019 10:07:57 -0700 (PDT) Subject: Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU To: Masami Hiramatsu Cc: Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Naresh Kamboju , Dan Rue , Matt Hart , Anders Roxell , Daniel Diaz , "Paul E . McKenney" References: <156378170297.12011.17385386326930403235.stgit@devnote2> <156378173770.12011.3832608237079432765.stgit@devnote2> From: James Morse Message-ID: <0290c71b-6ed3-c455-eb4a-3f6a670f5e37@arm.com> Date: Tue, 23 Jul 2019 18:07:56 +0100 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <156378173770.12011.3832608237079432765.stgit@devnote2> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 22/07/2019 08:48, Masami Hiramatsu wrote: > Make debug exceptions visible from RCU so that synchronize_rcu() > correctly track the debug exception handler. > > This also introduces sanity checks for user-mode exceptions as same > as x86's ist_enter()/ist_exit(). > > The debug exception can interrupt in idle task. For example, it warns > if we put a kprobe on a function called from idle task as below. > The warning message showed that the rcu_read_lock() caused this > problem. But actually, this means the RCU is lost the context which > is already in NMI/IRQ. > So make debug exception visible to RCU can fix this warning. > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > index 9568c116ac7f..a6b244240db6 100644 > --- a/arch/arm64/mm/fault.c > +++ b/arch/arm64/mm/fault.c > @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr, > debug_fault_info[nr].name = name; > } > > +/* > + * In debug exception context, we explicitly disable preemption. > + * This serves two purposes: it makes it much less likely that we would > + * accidentally schedule in exception context and it will force a warning > + * if we somehow manage to schedule by accident. > + */ > +static void debug_exception_enter(struct pt_regs *regs) > +{ > + if (user_mode(regs)) { > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU"); Would moving entry.S's context_tracking_user_exit() call to be before do_debug_exception() also fix this? I don't know the reason its done 'after' debug exception handling. Its always been like this: commit 6c81fe7925cc4c42 ("arm64: enable context tracking"). > + } else { > + /* > + * We might have interrupted pretty much anything. In > + * fact, if we're a debug exception, we can even interrupt > + * NMI processing. > + * We don't want in_nmi() to return true, > + * but we need to notify RCU. How come? If you interrupted an SError or pseudo-nmi, it already is. Those paths should all be painted no-kprobe, but I'm sure there are gaps. The hw-breakpoints can almost certainly hook them. > + */ > + rcu_nmi_enter(); Can we interrupt printk()? Do we need printk_nmi_enter()? ... What about ftrace? Because SError and pseudo-nmi can interrupt interrupt-masked code, we describe them as NMI. The only difference here is these exceptions are synchronous. I suspect we should make these debug exceptions nmi for EL1. We can then use this for the kprobe-re-entrance stuff so the pre/post hooks don't get run if they interrupted something also described as NMI. > + } > + > + preempt_disable(); > + > + /* This code is a bit fragile. Test it. */ > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work"); > +} > +NOKPROBE_SYMBOL(debug_exception_enter); Thanks, James