Re: [RFC][PATCH] IRQ: Fix oneshot irq race between irq_finalize_oneshot and handle_level_irq

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 9 Mar 2010, Valdis.Kletnieks@vt.edu wrote:
> On Tue, 09 Mar 2010 08:58:11 +0100, Thomas Gleixner said:
> > On Tue, 9 Mar 2010, Lars-Peter Clausen wrote:
> > 
> > > If the kernel has been compiled with preemtion support and handle_level_irq is
> > > called from process context for a oneshot irq there is a race between
> > > irq_finalize_oneshot and handle_level_irq which results in the irq not being
> > > unmasked after its handlers have been run.
> > > 
> > > irq_finalize_oneshot is expected to unmask the irq after the threaded irq
> > > handler has been run. It only does so if IRQ_MASKED is set for the irqs status.
> > > IRQ_MASKED gets set in the lower part of handle_level_irq after handle_IRQ_event
> > > has been called.
> > > handle_IRQ_event will wakeup the oneshot irqs threaded handler and if the
> > > kernel has been build with preemption there is a chance that the threaded irq
> > > handler will finish before execution is returned to handle_level_irq.
> > > As a result irq_finalize_oneshot will not unmask the irq and handle_level_irq
> > > will set the IRQ_MASKED flag. Thus the irq will stay masked and stalls.
> > > 
> > > In case of an race the call-graph would look like this:
> > >  handle_level_irq
> > >  |- mask_ack_irq
> > >  |- handle_IRQ_event
> > >     |- wake_up_process
> > >        |- irq_thread
> > >           |- action->thread_fn
> > >           |- irq_finalize_oneshot # Does not unmask the irq
> > > |- # Set IRQ_MASKED status flag
> > 
> > Errm, a thread _CANNOT_ preempt a hard interrupt handler.
> 
> What stops the thread from concurrently running on another CPU and racing
> that way?  I'm an idiot, use small words. :)

Right it's a valid SMP problem, but I got confused by the lengthy
explanation of a thread preempting an hard interrupt handler. :)

Will have a look after dinner.

Thanks,

	tglx