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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,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 5382EC34047 for ; Tue, 18 Feb 2020 20:40:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 292DE2173E for ; Tue, 18 Feb 2020 20:40:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="n7e0v8ss" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726922AbgBRUkl (ORCPT ); Tue, 18 Feb 2020 15:40:41 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:44366 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726384AbgBRUkl (ORCPT ); Tue, 18 Feb 2020 15:40:41 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=ezXVEe4FeJgs8vGaFryOelcymimRB2XjeyUjWso/aEk=; b=n7e0v8ssBmvuZIzr1HNNYBa55m ITiL3kfA2kGf72KEH3nMjzWivzsv1i2PW5dLib4/5l9Lm7H0ECtL/EpW91EticmO/Lgpx3Ma2FRAR 1MyD7qiac65G46JG0wqpVlA1YlxU9Nl8vNvtHZLrjv/oLbjsDsTp8BHkXfbv6RJuAkgFM7zLoqNZR HeLWLvjJf9AtNeaZq/eYam5p2hpMFT+s0UzRsnyuIRCq2fEzMBj0T+MlaaOfHNx4kNptTKHqvzjUg BQsn6vDPI3xEfRot3ghhf0udYMAH4LzYNx6AqsLNocLUpNZtsDwee+wBKvFqZzrYR8G68VSvb19Ot CQXNW+yw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j49fL-0000wX-0s; Tue, 18 Feb 2020 20:40:23 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 35813980E56; Tue, 18 Feb 2020 21:40:21 +0100 (CET) Date: Tue, 18 Feb 2020 21:40:21 +0100 From: Peter Zijlstra To: "Paul E. McKenney" 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: <20200218204021.GJ11457@worktop.programming.kicks-ass.net> References: <20200212210139.382424693@infradead.org> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218201728.GH2935@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) 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 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.