Return-path: Received: from mail.candelatech.com ([208.74.158.172]:40643 "EHLO ns3.lanforge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755944Ab3FEVdn (ORCPT ); Wed, 5 Jun 2013 17:33:43 -0400 Message-ID: <51AFAE94.5030007@candelatech.com> (sfid-20130605_233400_259087_4A0F314E) Date: Wed, 05 Jun 2013 14:33:08 -0700 From: Ben Greear MIME-Version: 1.0 To: Tejun Heo CC: Rusty Russell , Joe Lawrence , Linux Kernel Mailing List , stable@vger.kernel.org, "Luis R. Rodriguez" , Jouni Malinen , Vasanthakumar Thiagarajan , Senthil Balasubramanian , linux-wireless@vger.kernel.org, ath9k-devel@venema.h4ckr.net, Thomas Gleixner , Ingo Molnar Subject: Re: stop_machine lockup issue in 3.9.y. References: <20130604100744.7cdf8777@jlaw-desktop.mno.stratus.com> <51AE1B81.20900@candelatech.com> <51AE27D5.7050202@candelatech.com> <87sj0xry1k.fsf@rustcorp.com.au> <20130605071539.GA3429@mtj.dyndns.org> <51AF6E54.3050108@candelatech.com> <20130605184807.GD10693@mtj.dyndns.org> <51AF8D4B.4090407@candelatech.com> <51AF91F5.6090801@candelatech.com> <51AFA677.9010605@candelatech.com> <20130605211157.GK10693@mtj.dyndns.org> In-Reply-To: <20130605211157.GK10693@mtj.dyndns.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Sender: linux-wireless-owner@vger.kernel.org List-ID: On 06/05/2013 02:11 PM, Tejun Heo wrote: > (cc'ing wireless crowd, tglx and Ingo. The original thread is at > http://thread.gmane.org/gmane.linux.kernel/1500158/focus=55005 ) > > Hello, Ben. > > On Wed, Jun 05, 2013 at 01:58:31PM -0700, Ben Greear wrote: >> Hmm, wonder if I found it. I previously saw times where it appears >> jiffies does not increment. __do_softirq has a break-out based on >> jiffies timeout. Maybe that is failing to get us out of __do_softirq >> in my lockup case because for whatever reason the system cannot update >> jiffies in this case? >> >> I added this (probably whitespace damaged) hack and now I have not been >> able to reproduce the problem. > > Ah, nice catch. :) > >> diff --git a/kernel/softirq.c b/kernel/softirq.c >> index 14d7758..621ea3b 100644 >> --- a/kernel/softirq.c >> +++ b/kernel/softirq.c >> @@ -212,6 +212,7 @@ asmlinkage void __do_softirq(void) >> unsigned long end = jiffies + MAX_SOFTIRQ_TIME; >> int cpu; >> unsigned long old_flags = current->flags; >> + unsigned long loops = 0; >> >> /* >> * Mask out PF_MEMALLOC s current task context is borrowed for the >> @@ -241,6 +242,7 @@ restart: >> unsigned int vec_nr = h - softirq_vec; >> int prev_count = preempt_count(); >> >> + loops++; >> kstat_incr_softirqs_this_cpu(vec_nr); >> >> trace_softirq_entry(vec_nr); >> @@ -265,7 +267,7 @@ restart: >> >> pending = local_softirq_pending(); >> if (pending) { >> - if (time_before(jiffies, end) && !need_resched()) >> + if (time_before(jiffies, end) && !need_resched() && (loops < 500)) >> goto restart; > > So, softirq most likely kicked off from ath9k is rescheduling itself > to the extent where it ends up locking out the CPU completely. The > problem is usually okay because the processing would break out in 2ms > but as jiffies is stopped in this case with all other CPUs trapped in > stop_machine, the loop never breaks and the machine hangs. While > adding the counter limit probably isn't a bad idea, softirq requeueing > itself indefinitely sounds pretty buggy. Just to be clear on the ath9k part for the wifi folks: This is basically un-patched 3.9.4, but I have 200 virtual stations configured on each of two ath9k radios. I cannot reproduce the problem without ath9k, but I do not know for certain ath9k is the real culprit. In the case where I can most easily reproduce the lockup, ath9k virtual stations would be trying to associate, so I'd expect a fair amount of packet processing to be happening... > ath9k people, do you guys have any idea what's going on? Why would > softirq repeat itself indefinitely? > > Ingo, Thomas, we're seeing a stop_machine hanging because > > * All other CPUs entered IRQ disabled stage. Jiffies is not being > updated. > > * The last CPU get caught up executing softirq indefinitely. As > jiffies doesn't get updated, it never breaks out of softirq > handling. This is a deadlock. This CPU won't break out of softirq > handling unless jiffies is updated and other CPUs can't do anything > until this CPU enters the same stop_machine stage. > > Ben found out that breaking out of softirq handling after certain > number of repetitions makes the issue go away, which isn't a proper > fix but we might want anyway. What do you guys think? Thanks, Ben -- Ben Greear Candela Technologies Inc http://www.candelatech.com