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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 9C005C433E9 for ; Thu, 28 Jan 2021 17:59:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6369464E1B for ; Thu, 28 Jan 2021 17:59:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229695AbhA1R7s (ORCPT ); Thu, 28 Jan 2021 12:59:48 -0500 Received: from mail.skyhub.de ([5.9.137.197]:47428 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229593AbhA1R63 (ORCPT ); Thu, 28 Jan 2021 12:58:29 -0500 Received: from zn.tnic (p200300ec2f0a4b00db608beaaba2adee.dip0.t-ipconnect.de [IPv6:2003:ec:2f0a:4b00:db60:8bea:aba2:adee]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id 9486D1EC058C; Thu, 28 Jan 2021 18:57:41 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1611856661; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=T/dOLZHqyfSXT8UHCHdkk516q/7r513CDtEdL9VJMe0=; b=JVi7ocDa2q2IATomaId3fpoiN/LDhxvbpNFODzgUofJsUdv9XlHFcEs2yeCpx2aVfSnBT6 kYv3Ge/CN1jzCfqg4JiD295LveLTvw/HA+3dytSHBOA7eVQQ+2UhIVJPZKreVrvGSFqbPU Eqn3p/nu89ZAnYf7vFUSR3thif+aylg= Date: Thu, 28 Jan 2021 18:57:35 +0100 From: Borislav Petkov To: "Luck, Tony" Cc: x86@kernel.org, Andrew Morton , Peter Zijlstra , Darren Hart , Andy Lutomirski , linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v5] x86/mce: Avoid infinite loop for copy from user recovery Message-ID: <20210128175735.GB2120@zn.tnic> References: <20210115193435.GA4663@agluck-desk2.amr.corp.intel.com> <20210115205103.GA5920@agluck-desk2.amr.corp.intel.com> <20210115232346.GA7967@agluck-desk2.amr.corp.intel.com> <20210119105632.GF27433@zn.tnic> <20210119235759.GA9970@agluck-desk2.amr.corp.intel.com> <20210120121812.GF825@zn.tnic> <20210121210959.GA10304@agluck-desk2.amr.corp.intel.com> <20210125225509.GA7149@agluck-desk2.amr.corp.intel.com> <20210126110314.GC6514@zn.tnic> <20210126223605.GA14355@agluck-desk2.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210126223605.GA14355@agluck-desk2.amr.corp.intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 26, 2021 at 02:36:05PM -0800, Luck, Tony wrote: > In some cases Linux might context switch to something else. Perhaps > this task even gets picked up by another CPU to run the task work > queued functions. But I imagine that the context switch should act > as a barrier ... shouldn't it? I'm given to understand that the #MC from user is likely to schedule and a context switch has a barrier character. > After a few cycles of the test injection to user mode, I saw an > overflow in the machine check bank. As if it hadn't been cleared > from the previous iteration ... This sounds weird. As if something else is happening which we haven't thought of yet... > When the tests were failing, code was on top of v5.11-rc3. Latest > experiments moved to -rc5. There's just a tracing fix from > PeterZ between rc3 and rc5 to mce/core.c: > > 737495361d44 ("x86/mce: Remove explicit/superfluous tracing") > > which doesn't appear to be a candidate for the problems I saw. Doesn't look like it. > This is the bit that changed during my detour using atomic_t mce_count. > I added the local variable to capture value from atomic_inc_return(), then > used it later, instead of a bunch of atomic_read() calls. > > I kept it this way because "if (count == 1)" is marginally easier to read > than "if (current->mce_count++ == 0)" Right. So still no explanation why it would fail before. ;-\ Crazy idea: if you still can reproduce on -rc3, you could bisect: i.e., if you apply the patch on -rc3 and it explodes and if you apply the same patch on -rc5 and it works, then that could be a start... Yeah, don't have a better idea here. :-\ -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette