From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934390AbdDGQxm (ORCPT ); Fri, 7 Apr 2017 12:53:42 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:44313 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934261AbdDGQxh (ORCPT ); Fri, 7 Apr 2017 12:53:37 -0400 Date: Fri, 7 Apr 2017 09:53:29 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton Subject: Re: [PATCH 6/5]rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work Reply-To: paulmck@linux.vnet.ibm.com References: <20170407140106.051135969@goodmis.org> <20170407123516.0560f48c@gandalf.local.home> <20170407164220.GY1600@linux.vnet.ibm.com> <20170407124440.7697e14f@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170407124440.7697e14f@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17040716-0036-0000-0000-000001D4A31B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006894; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00844350; UDB=6.00416195; IPR=6.00622647; BA=6.00005275; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00014953; XFM=3.00000013; UTC=2017-04-07 16:53:31 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17040716-0037-0000-0000-00003F879D16 Message-Id: <20170407165329.GZ1600@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-07_15:,, 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=8.0.1-1702020001 definitions=main-1704070138 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 07, 2017 at 12:44:40PM -0400, Steven Rostedt wrote: > On Fri, 7 Apr 2017 09:42:20 -0700 > "Paul E. McKenney" wrote: > > > On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" > > > > > > Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when > > > it needs to use rcu_read_lock() and friends. This is because tracing can > > > happen as RCU is about to enter user space, or about to go idle, and RCU > > > does not watch for RCU read side critical sections as it makes the > > > transition. > > > > > > There is a small location within the RCU infrastructure that rcu_irq_enter() > > > itself will not work. If tracing were to occur in that section it will break > > > if it tries to use rcu_irq_enter(). > > > > > > Originally, this happens with the stack_tracer, because it will call > > > save_stack_trace when it encounters stack usage that is greater than any > > > stack usage it had encountered previously. There was a case where that > > > happened in the RCU section where rcu_irq_enter() did not work, and lockdep > > > complained loudly about it. To fix it, stack tracing added a call to be > > > disabled and RCU would disable stack tracing during the critical section > > > that rcu_irq_enter() was inoperable. This solution worked, but there are > > > other cases that use rcu_irq_enter() and it would be a good idea to let RCU > > > give a way to let others know that rcu_irq_enter() will not work. For > > > example, in trace events. > > > > > > Another helpful aspect of this change is that it also moves the per cpu > > > variable called in the RCU critical section into a cache locale along with > > > other RCU per cpu variables used in that same location. > > > > > > I'm keeping the stack_trace_disable() code, as that still could be used in > > > the future by places that really need to disable it. And since it's only a > > > static inline, it wont take up any kernel text if it is not used. > > > > > > Signed-off-by: Steven Rostedt (VMware) > > > > Looks better, especially __this_cpu_inc() and __this_cpu_dec(). > > > > How about rcu_irq_enter_disabled instead of rcu_disabled? We aren't > > really disabling all of RCU. ;-) > > OK, I'll make the update and send a 6.1 (and also a new 7/5)! So the ex-Borg member was really 9 of 7 rather than 7 of 9, then? ;-) Thanx, Paul