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

[Pekka Riikonen]

On Thu, 6 Jun 2013, Tejun Heo wrote:

> On Thu, Jun 06, 2013 at 02:29:49PM -0700, greearb@xxxxxxxxxxxxxxx wrote:
> > From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx>
>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> 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
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