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=-2.1 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 E01CAC11D00 for ; Thu, 20 Feb 2020 07:40:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AF23E2465D for ; Thu, 20 Feb 2020 07:40:20 +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="2CTht/UP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726826AbgBTHkT (ORCPT ); Thu, 20 Feb 2020 02:40:19 -0500 Received: from merlin.infradead.org ([205.233.59.134]:32872 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726756AbgBTHkS (ORCPT ); Thu, 20 Feb 2020 02:40:18 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=TtE3mlL5t0f9qnTzN7/JiHJ/6OUI4BCT1mqa2/farcY=; b=2CTht/UP19chea61+8j2T0TbdB gmcz+vagWKsoDDg5x/HshwlgWGMnt5LofDcxxWMqkbTSefaRXyZvA8ADzB3Q7ayxHACgFhGq4vMhd qIG/5mANVsp25mEC/YPw3csAfpSpyhS+o++k+4Hg37dmX1nqVf3q/m/B3HPE/aoP1UyJ+QJEk5wyS CRbGb/a3U9WM2OmfjuWJze2L8g1Q/jf3EW129qQPVexfAbPqON5sPLuHIQkWGNi93lBalOZCfLDpQ um7Kdmxrrkoe8hmT9VCSqYzdfjbgtPbApBxiOqqYVF+mCct/J6+YkMg3u5mA6/zCq2pFMneSAGQug 6iUsIfrA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j4gQb-0007Nh-Jz; Thu, 20 Feb 2020 07:39:21 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 69E16305E21; Thu, 20 Feb 2020 08:37:25 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 25E5120AFA9A9; Thu, 20 Feb 2020 08:39:18 +0100 (CET) Date: Thu, 20 Feb 2020 08:39:18 +0100 From: Peter Zijlstra To: Andy Lutomirski Cc: Borislav Petkov , LKML , linux-arch , Steven Rostedt , Ingo Molnar , Joel Fernandes , Greg KH , gustavo@embeddedor.com, Thomas Gleixner , paulmck@kernel.org, Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , Tony Luck , Frederic Weisbecker , Dan Carpenter , Masami Hiramatsu Subject: Re: [PATCH v3 02/22] x86,mce: Delete ist_begin_non_atomic() Message-ID: <20200220073918.GR18400@hirez.programming.kicks-ass.net> References: <20200219144724.800607165@infradead.org> <20200219150744.488895196@infradead.org> <20200219171309.GC32346@zn.tnic> <20200219173358.GP18400@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Wed, Feb 19, 2020 at 02:12:13PM -0800, Andy Lutomirski wrote: > On Wed, Feb 19, 2020 at 9:34 AM Peter Zijlstra wrote: > > Do you really want to create code that unwinds enough of nmi_enter() to > > get you to a preemptible context? *shudder* > > Well, there's another way to approach this: > > void notrace nonothing do_machine_check(struct pt_regs *regs) > { > if (user_mode(regs)) > do_sane_machine_check(regs); > else > do_awful_machine_check(regs); > } > > void do_sane_machine_check(regs) > { > nothing special here. just a regular exception, more or less. > } > > void do_awful_macine_check(regs) > { > basically an NMI. No funny business, no recovery possible. > task_work_add() not allowed. > } Right, that looks like major surgery to the current code though; I'd much prefer someone that knows that code do that. > I suppose the general consideration I'm trying to get at is: is > task_work_add() actually useful at all here? For the case when a > kernel thread does memcpy_mcsafe() or similar, task work registered > using task_work_add() will never run. task_work isn't at all useful when we didn't come from userspace. In that case irq_work is the best option, but that doesn't provide a preemptible context.