Re: [PATCH v3] Fix lockup related to stop_machine being stuck in __do_softirq.

[Pekka Riikonen <priikone@iki.fi>]

On Thu, 6 Jun 2013, Tejun Heo wrote:

> On Thu, Jun 06, 2013 at 02:29:49PM -0700, greearb@candelatech.com wrote:
>> From: Ben Greear <greearb@candelatech.com>
>>
>> The stop machine logic can lock up if all but one of
>> the migration threads make it through the disable-irq
>> step and the one remaining thread gets stuck in
>> __do_softirq.  The reason __do_softirq can hang is
>> that it has a bail-out based on jiffies timeout, but
>> in the lockup case, jiffies itself is not incremented.
>>
>> To work around this, re-add the max_restart counter in __do_irq
>> and stop processing irqs after 10 restarts.
>>
>> Thanks to Tejun Heo and Rusty Russell and others for
>> helping me track this down.
>>
>> This was introduced in 3.9 by commit:  c10d73671ad30f5469
>> (softirq:  reduce latencies).
>>
>> It may be worth looking into ath9k to see if it has issues with
>> it's irq handler at a later date.
>>
>> The hang stack traces look something like this:
> ...
>> Signed-off-by: Ben Greear <greearb@candelatech.com>
>
> Acked-by: Tejun Heo <tj@kernel.org>
>
> Linus, while this doesn't fix the root cause of the problem - softirq
> runaway - I still think this is a worthwhile protection to have.  Ben
> is in the process of finding out why the softirq runaway happens in
> the first place.  We probably want to add Cc: stable@vger.kernel.org
> tag.
>
The counter also helps to keep the interrupted task interrupted a shorter 
period of time.  10 iterations may be a lot shorter than the 2 ms, or 10 
ms with HZ=100, so it helps interactivity also.  This is a good change 
to bring back in any case.

 	Pekka