From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757379AbdERSph (ORCPT ); Thu, 18 May 2017 14:45:37 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:47056 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757367AbdERSpd (ORCPT ); Thu, 18 May 2017 14:45:33 -0400 Date: Thu, 18 May 2017 11:45:28 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: LKML , Ingo Molnar , Thomas Gleixner Subject: Re: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace Reply-To: paulmck@linux.vnet.ibm.com References: <20170518093809.13428efa@gandalf.local.home> <20170518154711.GR3956@linux.vnet.ibm.com> <20170518123914.689a2b02@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170518123914.689a2b02@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17051818-0036-0000-0000-0000020B1C71 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007081; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00862321; UDB=6.00427790; IPR=6.00641980; BA=6.00005358; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015501; XFM=3.00000015; UTC=2017-05-18 18:45:30 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17051818-0037-0000-0000-0000406337CF Message-Id: <20170518184528.GV3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-05-18_04:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1705180126 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul > -- Steve > > > > Thanx, Paul > > > > > + __ftrace_trace_stack(buffer, flags, skip, pc, NULL); > > > + return; > > > + } > > > + > > > + /* > > > + * It is possible that a function is being traced in a > > > + * location that RCU is not watching. A call to > > > + * rcu_irq_enter() will make sure that it is, but there's > > > + * a few internal rcu functions that could be traced > > > + * where that wont work either. In those cases, we just > > > + * do nothing. > > > + */ > > > + if (unlikely(rcu_irq_enter_disabled())) > > > + return; > > > + > > > + rcu_irq_enter(); > > > + __ftrace_trace_stack(buffer, flags, skip, pc, NULL); > > > + rcu_irq_exit(); > > > } > > > > > > /** > > > -- > > > 2.9.3 > > > >