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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham 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 C5AF3C43142 for ; Mon, 25 Jun 2018 16:39:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7E31D2578C for ; Mon, 25 Jun 2018 16:39:58 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="pBl4tbPL" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7E31D2578C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755310AbeFYQj4 (ORCPT ); Mon, 25 Jun 2018 12:39:56 -0400 Received: from mail-pg0-f66.google.com ([74.125.83.66]:33629 "EHLO mail-pg0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752582AbeFYQjx (ORCPT ); Mon, 25 Jun 2018 12:39:53 -0400 Received: by mail-pg0-f66.google.com with SMTP id e11-v6so6312422pgq.0 for ; Mon, 25 Jun 2018 09:39:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=07eJdvkwK+7ZIL+hMaWZ7lVgd8BCW3xW00fovHnBhzA=; b=pBl4tbPLaF0YPMXGetfoy4ugfvdoyGEH+HOCE/0p3D1aI4pTiT781X0ZH5RAGl4VY4 l/wDJN3mYTsN7dPGNn1MmxE3Dtd3ZPSoL9VH7Aej8Fp2opOifmuTp/xx4PFYbHkNnrmE F+hyqcjlWpE6u1eEPfYk2rregPiYLnomJXJy4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=07eJdvkwK+7ZIL+hMaWZ7lVgd8BCW3xW00fovHnBhzA=; b=e7tXIePk6+Qq+UXzJ4a5hWnP8zWmBOl95OZ8x4Bnwvwkw0WvNYxfAoRVGpyqMSYuwQ NpzTUVITD+y3gx5hgMd7L5ygB/lrpbwWx+7VUimg4K1ZPxfd0OOUfMg9enhcYY50bpbj yWjjdrutB5bv7b2ad6jp7vGPC6DKcll5BcYG5kDVJadJSu5pgnf0n+PtTmwNWK1CpuAw hn72QBJEP93lZ3gQ21xvb9nBekk5ObrGIlDpsD0b4+QyQDIR7aJi8xYaCpgmaRawa/aQ sJBGE1krSa1IzOMe5idzmidoHGdVJXyumeHbUFZqO8a/eMxRQRAboYdBMHRBaRUiA3Z0 7z4w== X-Gm-Message-State: APt69E0lyNhI2wB/CYqfGN4f6Opem7mbnaOg1TfL3Y0Zdof/qYRmpAtQ G5eY9oAtXR8E4vAk99NbL/m5Vg== X-Google-Smtp-Source: ADUXVKJqqZtEgrLSm1x+21qGUnKOoQka9gXVVc7VfCP4lWQEVYcF7ft7eM0gVPfrwsUYBzmz1X0KMg== X-Received: by 2002:a63:8c5:: with SMTP id 188-v6mr11213200pgi.97.1529944793393; Mon, 25 Jun 2018 09:39:53 -0700 (PDT) Received: from localhost ([2620:0:1000:1600:3122:ea9c:d178:eb]) by smtp.gmail.com with ESMTPSA id a1-v6sm19298761pgu.81.2018.06.25.09.39.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 25 Jun 2018 09:39:52 -0700 (PDT) Date: Mon, 25 Jun 2018 09:39:51 -0700 From: Joel Fernandes To: Byungchul Park Cc: Steven Rostedt , "Paul E. McKenney" , Byungchul Park , jiangshanlai@gmail.com, josh@joshtriplett.org, Mathieu Desnoyers , linux-kernel@vger.kernel.org, kernel-team@lge.com, luto@kernel.org Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks Message-ID: <20180625163951.GA52646@joelaf.mtv.corp.google.com> References: <1529484440-20634-2-git-send-email-byungchul.park@lge.com> <20180620145814.GQ3593@linux.vnet.ibm.com> <20180620164902.GW3593@linux.vnet.ibm.com> <20180622055659.GA255098@joelaf.mtv.corp.google.com> <20180622132843.GN3593@linux.vnet.ibm.com> <20180622181916.GA13628@joelaf.mtv.corp.google.com> <20180622143247.781028b1@gandalf.local.home> <20180622200548.GA114655@joelaf.mtv.corp.google.com> <20180625082824.GB21377@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180625082824.GB21377@X58A-UD3R> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 25, 2018 at 05:28:24PM +0900, Byungchul Park wrote: > On Fri, Jun 22, 2018 at 01:05:48PM -0700, Joel Fernandes wrote: > > On Fri, Jun 22, 2018 at 02:32:47PM -0400, Steven Rostedt wrote: > > > On Fri, 22 Jun 2018 11:19:16 -0700 > > > Joel Fernandes wrote: > > > > > > > Sure. So in a later thread you mentioned "usermode helpers". I took a closer > > > > look at that subsystem, and it seems you can execute usermode helpers from > > > > atomic sections with help of UMH_NO_WAIT flag. > > > > > > > > Then I checked where this flag is used and it turns out its from the > > > > mce_work_trigger function in x86/kernel/cpu/mcheck/dev-mcelog.c which can be > > > > called infact from an interrupt context (mce_notify_irq). > > > > > > > > Is this the usecase you remember causing this weird transitions to userspace? > > > > > > But this case still looks like it uses work queues, it just doesn't > > > wait for the result. > > > > > > I'll have to look at the code from what it looked like back in 2011, to > > > see if there was an actual issue here back then. > > > > Good point Steve. So I guess in the current kernel sources, there's no code > > that uses UMH in IRQ context AFAICT. I'll go through the google group thread > > Paul pointed as well to study the history of the problem a bit more. > > Me too. Good discussion we had thanks to you, Joel. No problem, thanks for the patch in the first place which triggered this discussion. For whatever its worth, I made some notes of what I understood from reading the code and old posts because I was sure I would otherwise forget everything: http://www.joelfernandes.org/linuxinternals/2018/06/15/rcu-dynticks.html Feel free to comment on that post directly (or here) if you feel something is grossly wrong. Again thank you and everyone for the discussion! ;-) - Joel