Re: rt20 patch question

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> Ingo,
> 
> I traced this down.  It is caused by the disable_irq in vortex_timer 
> that is called via run_timer_softirq.
> 
> disable_irq can call synchronize_irq which can schedule.
> 
> And thus you get this bug since we are in a softirq.

hm. When there are threaded interrupts, we quite naturally have to 
synchronize via scheduling, in synchronize_irq() - the interrupt we are 
waiting on might be scheduled away!

> So I guess we have a case that we can schedule, but while atomic and 
> BUG when it's really not bad.  Should we add something like this:

that's not good enough, we must not schedule with the preempt_count() 
set.

one solution would be to forbid disable_irq() from softirq contexts, and 
to convert the vortex timeout function to a workqueue and use the 
*_delayed_work() APIs to drive it - and cross fingers there's not many 
places to fix.

another solution would be to make softirqs preemptible if they are 
threaded. I'm a bit uneasy about that though. In that case we'd also 
have to make HARDIRQ threading dependent on softirq threading, in the 
Kconfig.

	Ingo