From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756370AbdERTab (ORCPT ); Thu, 18 May 2017 15:30:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:38816 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754449AbdERTaa (ORCPT ); Thu, 18 May 2017 15:30:30 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C91FD239BA Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=rostedt@goodmis.org Date: Thu, 18 May 2017 15:30:26 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: LKML , Ingo Molnar , Thomas Gleixner Subject: Re: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace Message-ID: <20170518153026.053b2c82@gandalf.local.home> In-Reply-To: <20170518184528.GV3956@linux.vnet.ibm.com> References: <20170518093809.13428efa@gandalf.local.home> <20170518154711.GR3956@linux.vnet.ibm.com> <20170518123914.689a2b02@gandalf.local.home> <20170518184528.GV3956@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 18 May 2017 11:45:28 -0700 "Paul E. McKenney" wrote: > On Thu, May 18, 2017 at 12:39:14PM -0400, Steven Rostedt wrote: > > On Thu, 18 May 2017 08:47:11 -0700 > > "Paul E. McKenney" wrote: > > > > > On Thu, May 18, 2017 at 09:38:09AM -0400, Steven Rostedt wrote: > > > > > > > > From: "Steven Rostedt (VMware)" > > > > > > > > As stack tracing now requires "rcu watching", force RCU to be watching when > > > > recording a stack trace. > > > > > > > > Link: http://lkml.kernel.org/r/20170512172449.879684501@goodmis.org > > > > > > > > Cc: "Paul E. McKenney" > > > > Signed-off-by: Steven Rostedt (VMware) > > > > --- > > > > > > > > Changes since v1: > > > > > > > > My testing discovered that the stack trace can be called with > > > > interrupts enabled, which is a no no to have when calling > > > > rcu_irq_enter(). When interrupts are enabled, as with being in an > > > > NMI, RCU will also be watching. > > > > > > > > kernel/trace/trace.c | 26 +++++++++++++++++++++++++- > > > > 1 file changed, 25 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > > > > index fcc9a2d..34a98ba 100644 > > > > --- a/kernel/trace/trace.c > > > > +++ b/kernel/trace/trace.c > > > > @@ -2568,7 +2568,31 @@ static inline void ftrace_trace_stack(struct trace_array *tr, > > > > void __trace_stack(struct trace_array *tr, unsigned long flags, int skip, > > > > int pc) > > > > { > > > > - __ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL); > > > > + struct ring_buffer *buffer = tr->trace_buffer.buffer; > > > > + > > > > + /* > > > > + * When an NMI triggers, RCU is enabled via rcu_nmi_enter() > > > > + * Also, RCU is always enabled when interrupts are. > > > > + */ > > > > + if (!irqs_disabled() || in_nmi()) { > > > > > > You lost me on this one. RCU might not be watching if irqs are > > > enabled, for example, in the idle loop. What am I missing here? > > > > > > > Hmm, no, maybe I'm missing something :-/ > > > > OK, so if we trace in the idle loop, rcu may not be watching, so I may > > need to byte the bullet and use the rcu_irq_enter_irqon() instead :-( > > If rcu_is_watching() returns false, yes. Of course, if rcu_is_watching() > returns true, you are golden. > Hmm, so I can just replace the entire if with: if (rcu_is_watching()) because it should be watching in NMI context, correct? -- Steve