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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 8815CC34031 for ; Tue, 18 Feb 2020 21:39:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5C55524125 for ; Tue, 18 Feb 2020 21:39:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582061968; bh=k/tBVL6eIxoxj8oKzTUujuA9bmvqwEW41wa8tE6sUps=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=Zla/4AF3cLenqMH2ATViDVf5G7ngm+DApjPmBU2EHwQjldq4fAV8D4aQ6CmXIiTax kRljxmqZ25NSSKeRPqNSxqEmMeQT4tCKg9/uFQZmVe1H5739ea6G+VtUEhFS+zWgm1 ASpt91Oi2vWG72VJBglIWI2dxKKaonmOp+vxIJO8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727875AbgBRVj1 (ORCPT ); Tue, 18 Feb 2020 16:39:27 -0500 Received: from mail.kernel.org ([198.145.29.99]:47642 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726481AbgBRVj0 (ORCPT ); Tue, 18 Feb 2020 16:39:26 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 E3D362064C; Tue, 18 Feb 2020 21:39:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582061966; bh=k/tBVL6eIxoxj8oKzTUujuA9bmvqwEW41wa8tE6sUps=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Q84+4Y8OJYIS0OSkpyVPQhtqsUx7lfuDh9zip37wSaC4qbj8lEqkBMDoBt9MIlSyv L28J8gHpSuDoDWtMBTYupKRCMXwk2DPw+o0lc57RZEu81Dk3rteFUtxxGchO+up+oM mclqytuLuZCV2wzK0GL6L3CpL3rGMv+b5tvOxr+Q= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id BF0113520856; Tue, 18 Feb 2020 13:39:25 -0800 (PST) Date: Tue, 18 Feb 2020 13:39:25 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Joel Fernandes , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, rostedt@goodmis.org, mingo@kernel.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}() Message-ID: <20200218213925.GL2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200212210749.971717428@infradead.org> <20200212232005.GC115917@google.com> <20200213082716.GI14897@hirez.programming.kicks-ass.net> <20200213135138.GB2935@paulmck-ThinkPad-P72> <20200213164031.GH14914@hirez.programming.kicks-ass.net> <20200213185612.GG2935@paulmck-ThinkPad-P72> <20200213204444.GA94647@google.com> <20200218195831.GD11457@worktop.programming.kicks-ass.net> <20200218201728.GH2935@paulmck-ThinkPad-P72> <20200218204021.GJ11457@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218204021.GJ11457@worktop.programming.kicks-ass.net> 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 Tue, Feb 18, 2020 at 09:40:21PM +0100, Peter Zijlstra wrote: > On Tue, Feb 18, 2020 at 12:17:28PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 18, 2020 at 08:58:31PM +0100, Peter Zijlstra wrote: > > > On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote: > > > > > > > > > That _should_ already be the case today. That is, if we end up in a > > > > > > tracer and in_nmi() is unreliable we're already screwed anyway. > > > > > > > I removed the static from rcu_nmi_enter()/exit() as it is called from > > > > outside, that makes it build now. Updated below is Paul's diff. I also added > > > > NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed > > > > asymmetric. > > > > > > > +__always_inline void rcu_nmi_exit(void) > > > > { > > > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > > > > > > > @@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq) > > > > trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks)); > > > > WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */ > > > > > > > > - if (irq) > > > > + if (!in_nmi()) > > > > rcu_prepare_for_idle(); > > > > > > > > rcu_dynticks_eqs_enter(); > > > > > > > > - if (irq) > > > > + if (!in_nmi()) > > > > rcu_dynticks_task_enter(); > > > > } > > > > > > Boris and me have been going over the #MC code (and finding loads of > > > 'interesting' code) and ran into ist_enter(), whish has the following > > > code: > > > > > > /* > > > * We might have interrupted pretty much anything. In > > > * fact, if we're a machine check, we can even interrupt > > > * NMI processing. We don't want in_nmi() to return true, > > > * but we need to notify RCU. > > > */ > > > rcu_nmi_enter(); > > > > > > > > > Which, to me, sounds all sorts of broken. The IST (be it #DB or #MC) can > > > happen while we're holding all sorts of locks. This must be an NMI-like > > > context. > > > > Ouch! Looks like I need to hold off on getting rid of the "irq" > > parameters if in_nmi() isn't going to be accurate. > > I'm currently trying to twist my brain around all this, because I > suspect it's all completely broken one way or another. > > But yes, we definitely need to fix this before your patch goes in. OK. I will drop it later today, but leave tag in_nmi.2020.02.18a pointing at it for future reference. Thanx, Paul