Re: [RFC] [PATCH] Fix misrouted interrupts deadlocks

[Vivek Goyal <vgoyal@in.ibm.com>]

On Mon, Nov 20, 2006 at 02:56:52PM -0500, Vivek Goyal wrote:
> On Mon, Nov 20, 2006 at 02:23:35PM -0500, Vivek Goyal wrote:
> > On Fri, Nov 10, 2006 at 04:55:48PM +0300, Pavel Emelianov wrote:
> > > As the second lock on booth CPUs is taken before checking that
> > > this irq is being handled in another processor this may cause
> > > a deadlock. This issue is only theoretical.
> > > 
> > > I propose the attached patch to fix booth problems: when trying
> > > to handle misrouted IRQ active desc->lock may be unlocked.
> > > 
> > > Please comment.
> > 
> > > --- ./kernel/irq/spurious.c.irqlockup	2006-11-09 11:19:10.000000000 +0300
> > > +++ ./kernel/irq/spurious.c	2006-11-10 16:53:38.000000000 +0300
> > > @@ -147,7 +147,11 @@ void note_interrupt(unsigned int irq, st
> > >  	if (unlikely(irqfixup)) {
> > >  		/* Don't punish working computers */
> > >  		if ((irqfixup == 2 && irq == 0) || action_ret == IRQ_NONE) {
> > > -			int ok = misrouted_irq(irq);
> > > +			int ok;
> > > +
> > > +			spin_unlock(&desc->lock);
> > > +			ok = misrouted_irq(irq);
> > > +			spin_lock(&desc->lock);
> > 
> > Hi Pavel,
> > 
> > Till -rc5, I was able to boot a kernel with irqpoll option and in -rc6 I 
> > can't. It simply hangs. I suspect it is realted to this change. I have yet
> > to confirm that. But before that one observation.
> > 
> 
> Hi Pavel,
> 
> If I backout your changes, everything works fine. So it looks like that
> the problem I am facing is because of your patch but I don't have a logical
> explanation yet that why the problem is there. Just realasing a lock
> which is not currently acquired should not hang the system?
>
Some more data regarding this issue.

For my system it gets locked in following sequence.

handle_level_irq() {
	spin_unlock(desc->lock)
	......
	note_interrupt() {
		/* Called without desc->lock held */
		spin_unlock(desc->lock)
		misrouted_irq()
		spin_lock(desc->lock)	
	}
	spin_lock(desc->lock) /* Lockup */ 	
}

So basically problems seems to be due to calling conventions of
note_interrupt(). In few cases we call it with desc->lock held and in other
cases we call it with desc->lock not held.

I guess, note_interrupt() should restore the desc->lock status back to the 
state of the lock when function was entered so that caller does not lockup.

Thanks
Vivek
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