[ath9k-devel] stop_machine lockup issue in 3.9.y.

[Ben Greear <greearb@candelatech.com>]

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 <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com