From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}() Date: Thu, 13 Feb 2020 14:39:18 -0800 Message-ID: <20200213223918.GN2935@paulmck-ThinkPad-P72> References: <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> <20200213205442.GK2935@paulmck-ThinkPad-P72> <20200213211930.GG170680@google.com> <20200213163800.5c51a5f1@gandalf.local.home> <20200213215004.GM2935@paulmck-ThinkPad-P72> <20200213170451.690c4e5c@gandalf.local.home> Reply-To: paulmck@kernel.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail.kernel.org ([198.145.29.99]:37350 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727669AbgBMWjW (ORCPT ); Thu, 13 Feb 2020 17:39:22 -0500 Content-Disposition: inline In-Reply-To: <20200213170451.690c4e5c@gandalf.local.home> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Steven Rostedt Cc: 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, Masami Hiramatsu 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? Thanx, Paul