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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 2D077C34022 for ; Mon, 17 Feb 2020 16:31:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E95CC215A4 for ; Mon, 17 Feb 2020 16:31:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581957075; bh=geBmkKBvdkCW5/3hc5Ff7VhO4lSH27P7qmkAO5Wxr9g=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=hmWPLtJjVkEIooGii97P/Bq/PInhzWTLtZMb8f39PLgHU5wuiEtA1WTJYvhREvjvy V+bCmJI1gRECKh2JivomOYvDbQCb/13O8FMy9AQTukEcjpNp91UoXuFvQDL8s4IzDW d7AsOCqX4j6SrrqB9OXH2m01zA7R1ZAti0O1X8x8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728605AbgBQQbN (ORCPT ); Mon, 17 Feb 2020 11:31:13 -0500 Received: from mail.kernel.org ([198.145.29.99]:38044 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727922AbgBQQbN (ORCPT ); Mon, 17 Feb 2020 11:31:13 -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 9A7B0214D8; Mon, 17 Feb 2020 16:31:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581957072; bh=geBmkKBvdkCW5/3hc5Ff7VhO4lSH27P7qmkAO5Wxr9g=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=DAY6B7zUn+WVe0hHI23Stu+wQ8WvpYhKlEwAvDZriYWS0gYoQcv72U3QSG/9h/odv BPqpGLFjXpZBLedhzoKxHe09LEgiclI4WkTIkiIR6fokwXyT4zCAKQgq7fU0MRqFnT lbdE0I/ioM0pzv8j7aXkQ6Y/RZ8BZsWHhiChQf54= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7757735226F4; Mon, 17 Feb 2020 08:31:12 -0800 (PST) Date: Mon, 17 Feb 2020 08:31:12 -0800 From: "Paul E. McKenney" To: Masami Hiramatsu Cc: Steven Rostedt , Joel Fernandes , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.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: <20200217163112.GM2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200213204444.GA94647@google.com> <20200213205442.GK2935@paulmck-ThinkPad-P72> <20200213211930.GG170680@google.com> <20200213163800.5c51a5f1@gandalf.local.home> <20200213215004.GM2935@paulmck-ThinkPad-P72> <20200213170451.690c4e5c@gandalf.local.home> <20200213223918.GN2935@paulmck-ThinkPad-P72> <20200214151906.b1354a7ed6b01fc3bf2de862@kernel.org> <20200215145934.GD2935@paulmck-ThinkPad-P72> <20200217175519.12a694a969c1a8fb2e49905e@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200217175519.12a694a969c1a8fb2e49905e@kernel.org> 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 Mon, Feb 17, 2020 at 05:55:19PM +0900, Masami Hiramatsu wrote: > On Sat, 15 Feb 2020 06:59:34 -0800 > "Paul E. McKenney" wrote: > > > On Fri, Feb 14, 2020 at 03:19:06PM +0900, Masami Hiramatsu wrote: > > > On Thu, 13 Feb 2020 14:39:18 -0800 > > > "Paul E. McKenney" wrote: > > > > > > > On Thu, Feb 13, 2020 at 05:04:51PM -0500, Steven Rostedt wrote: > > > > > On Thu, 13 Feb 2020 13:50:04 -0800 > > > > > "Paul E. McKenney" wrote: > > > > > > > > > > > On Thu, Feb 13, 2020 at 04:38:25PM -0500, Steven Rostedt wrote: > > > > > > > [ Added Masami ] > > > > > > > > > > > > > > On Thu, 13 Feb 2020 16:19:30 -0500 > > > > > > > Joel Fernandes wrote: > > > > > > > > > > > > > > > On Thu, Feb 13, 2020 at 12:54:42PM -0800, Paul E. McKenney wrote: > > > > > > > > > On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote: > > > > > > > > > > On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote: > > > > > > > > > > [...] > > > > > > > > > > > > > It might well be that I could make these functions be NMI-safe, but > > > > > > > > > > > > > rcu_prepare_for_idle() in particular would be a bit ugly at best. > > > > > > > > > > > > > So, before looking into that, I have a question. Given these proposed > > > > > > > > > > > > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able > > > > > > > > > > > > > to just use in_nmi()? > > > > > > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > > > So something like this, then? This is untested, probably doesn't even > > > > > > > > > > > build, and could use some careful review from both Peter and Steve, > > > > > > > > > > > at least. As in the below is the second version of the patch, the first > > > > > > > > > > > having been missing a couple of important "!" characters. > > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > My compiler complained about the static and the __always_inline, so I > > > > > > > > > fixed those. But please help me out on adding the NOKPROBE_SYMBOL() > > > > > > > > > to rcu_nmi_exit(). What bad thing happens if we leave this on only > > > > > > > > > rcu_nmi_enter()? > > > > > > > > > > > > > > > > It seemed odd to me we were not allowing kprobe on the rcu_nmi_enter() but > > > > > > > > allowing it on exit (from a code reading standpoint) so my reaction was to > > > > > > > > add it to both, but we could probably keep that as a separate > > > > > > > > patch/discussion since it is slightly unrelated to the patch.. Sorry to > > > > > > > > confuse the topic. > > > > > > > > > > > > > > > > > > > > > > rcu_nmi_enter() was marked NOKPROBE or other reasons. See commit > > > > > > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before > > > > > > > kprobe_int3_handler()") > > > > > > > > > > > > > > The issue was that we must not allow anything in do_int3() call kprobe > > > > > > > code before kprobe_int3_handler() is called. Because ist_enter() (in > > > > > > > do_int3()) calls rcu_nmi_enter() it had to be marked NOKPROBE. It had > > > > > > > nothing to do with it being RCU nor NMI, but because it was simply > > > > > > > called in do_int3(). > > > > > > > > > > > > > > Thus, there's no reason to make rcu_nmi_exit() NOKPROBE. But a commont > > > > > > > to why rcu_nmi_enter() would probably be useful, like below: > > > > > > > > > > > > Thank you, Steve! Could I please have your Signed-off-by for this? > > > > > > > > > > Sure, but it was untested ;-) > > > > > > > > No problem! I will fire up rcutorture on it. ;-) > > > > > > > > But experience indicates that you cannot even make a joke around here. > > > > There is probably already someone out there somewhere building a > > > > comment-checker based on deep semantic analysis and machine learning. :-/ > > > > > > > > > Signed-off-by: Steven Rostedt (VMware) > > > > > > > > > > I'd like a Reviewed-by from Masami though. > > > > > > > > Sounds good! Masami, would you be willing to review? > > > > > > Yes, the functions before calling kprobe_int3_handler() must not > > > be kprobed. It can cause an infinite recursive int3 trapping. > > > > > > Reviewed-by: Masami Hiramatsu > > > > Thank you both! > > > > Like this? > > > > Thanx, Paul > > > > This is good to me. Thank you for looking it over! (I already have your > BTW, if you consider the x86 specific code is in the generic file, > we can move NOKPROBE_SYMBOL() in arch/x86/kernel/traps.c. > (Sorry, I've hit this idea right now) Might this affect other architectures with NMIs and probe-like things? If so, it might make sense to leave it where it is. Thanx, Paul > Thank you, > > > ------------------------------------------------------------------------ > > > > commit 1817fdc8f4e4bd18c76305c9b937fb0dccbb1583 > > Author: Steven Rostedt > > Date: Sat Feb 15 06:54:50 2020 -0800 > > > > rcu: Provide comment for NOKPROBE() on rcu_nmi_enter() > > > > The rcu_nmi_enter() function was marked NOKPROBE() by commit > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before > > kprobe_int3_handler()") because the do_int3() call kprobe code must > > not be invoked before kprobe_int3_handler() is called. It turns out > > that ist_enter() (in do_int3()) calls rcu_nmi_enter(), hence the > > marking NOKPROBE() being added to rcu_nmi_enter(). > > > > This commit therefore adds a comment documenting this line of reasoning. > > > > Signed-off-by: Steven Rostedt > > Reviewed-by: Masami Hiramatsu > > Signed-off-by: Paul E. McKenney > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 132b53e..4a885af 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -835,6 +835,12 @@ void rcu_nmi_enter(void) > > rdp->dynticks_nmi_nesting + incby); > > barrier(); > > } > > +/* > > + * On x86, All functions in do_int3() must be marked NOKPROBE before > > + * kprobe_int3_handler() is called. ist_enter() which is called in do_int3() > > + * before kprobe_int3_handle() happens to call rcu_nmi_enter() which means > > + * that rcu_nmi_enter() must be marked NOKRPOBE. > > + */ > > NOKPROBE_SYMBOL(rcu_nmi_enter); > > > > /** > > > -- > Masami Hiramatsu