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

From: Tejun Heo
Date: Thu Jun 06 2013 - 17:40:27 EST

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


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