Date: Thu, 6 Jun 2013 14:40:14 -0700
From: Tejun Heo <tj@kernel.org>
To: greearb@candelatech.com
Cc: linux-kernel@vger.kernel.org, eric.dumazet@gmail.com,
        stable@vger.kernel.org, torvalds@linux-foundation.org
Subject: Re: [PATCH v3] Fix lockup related to stop_machine being stuck in
 __do_softirq.
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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.

Thanks.

-- 
tejun
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