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=-4.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 80429C7618B for ; Wed, 24 Jul 2019 11:47:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 44CD9229ED for ; Wed, 24 Jul 2019 11:47:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563968876; bh=mBlNAmlgpbL8sIsWIS0MT/Km8ZKqFPisBuV/zTsyUU4=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=CMPWjJ12uH3V3nIMPD/2aCRvCXIBTYW7RuD3VbhWlWTRaXqp+Ga2J+npB+nGG2aCr tktiAnzKZ6G3Yv96gKOxyKnmPJ1Mvq8D6c92/pIFNptLtj/dNL5HoOwcCLjs7oX2W8 dWSSW23yscv56kMupjbbzNvGQ6v3ieiELn7XWni0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727822AbfGXLry (ORCPT ); Wed, 24 Jul 2019 07:47:54 -0400 Received: from mail.kernel.org ([198.145.29.99]:32772 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727128AbfGXLry (ORCPT ); Wed, 24 Jul 2019 07:47:54 -0400 Received: from devnote2 (NE2965lan1.rev.em-net.ne.jp [210.141.244.193]) (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 C506122387; Wed, 24 Jul 2019 11:47:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563968873; bh=mBlNAmlgpbL8sIsWIS0MT/Km8ZKqFPisBuV/zTsyUU4=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=fW20VpN2srzvHeaON2aO9PX860hcw585Lgh8gjeSOCIXozBFl1aXstLO3FISoddd2 mCm+LcnzB1FUNo1wCMfGn3zsJWmTU/ozny13dklCz3DHrxCfJ0X7lJM8EndoFPBA7d l8KF7HLC8T2tQa5sTG83bBoq/dkligeH2bI43MP8= Date: Wed, 24 Jul 2019 20:47:48 +0900 From: Masami Hiramatsu To: James Morse 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" Subject: Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU Message-Id: <20190724204748.94abebef1f184032d2e77f73@kernel.org> In-Reply-To: <0290c71b-6ed3-c455-eb4a-3f6a670f5e37@arm.com> References: <156378170297.12011.17385386326930403235.stgit@devnote2> <156378173770.12011.3832608237079432765.stgit@devnote2> <0290c71b-6ed3-c455-eb4a-3f6a670f5e37@arm.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 23 Jul 2019 18:07:56 +0100 James Morse wrote: > 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? It sounds like treating only user context, correct? This part is just adding assertion, not fixing the problem which Naresh reported. > > 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. I think that sentense means "we don't want that this code makes in_nmi() to return true" So, if the breakpoint interrupts pNMI/SError context, it is OK that in_nmi() returns true. > > > > + */ > > + rcu_nmi_enter(); > > Can we interrupt printk()? Do we need printk_nmi_enter()? ... What about ftrace? Good point! As far as I know, we don't use it because ftrace doesn't use printk. But indeed, kprobes user can use printk and they have to call printk_nmi_enter()/exit(), that must be commented in the documentation. Anyway, basically it is user's choice. > > 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. I'm not sure how it can prevent... anyway because we have to run a single-stepping for recovery, and kprobe already check the reentered kprobes and skip user-handlers in such case. Thank you, > > > > + } > > + > > + 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 -- Masami Hiramatsu