Re: [PATCH 3/3] do_exit(): do not panic if exiting thread is not serving an interrupt

[Alexander Gordeev <agordeev@redhat.com>]

On Thu, Mar 22, 2012 at 12:56:55PM +0100, Thomas Gleixner wrote:
> On Mon, 19 Mar 2012, Alexander Gordeev wrote:
> 
> > Currently a crashed and killed forced oneshot threaded handler hits
> > in_interrupt() check in do_exit() and panics. As result, the code that
> > cleans up IRQ descriptor never not get called and IRQ line stays masked.
> > 
> > Similarly non-forced oneshot threaded handlers that crashed while holding
> > bh lock leave a IRQ line masked.
> > 
> > Regular threaded handlers that crashed while holding bh simply panic,
> > although they could have just terminate loudly.
> > 
> > This fix allows IRQ threaded handlers get killed gracefully instead of
> > panicking.
> > 
> > Since introduction of SOFTIRQ_DISABLE_OFFSET in 75e1056 we can differ
> > between bh being serviced and bh being disabled. Use this ability to
> > avoid unnecessary crashes when a exiting thread explicitly disabled bh
> > and is not serving any softirq. Still we will get the regular warning
> > that exiting thread is in atomic context.
> 
> Hmm, this applies for all threads which exit with bh disabled. We risk
> data corruption this way as the crash of a task might happen within a
> data set manipulation protected by bh_disable.

True. But we live with this as we do exit with preemption disabled. Are bh are
terribly different in this regard?

Anyway, I do not have strong opinion here. My point is letting innocent devices
on the shared irq line to go on worth considering.

> Not sure whether the chance to get debug information from the machine
> is worth the risk of data corruption causes follow up problems.

I would judge: no ;)

>  
> Thanks,
> 
> 	tglx
> 
> 
> > Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
> > ---
> >  include/linux/hardirq.h |    4 ++++
> >  kernel/exit.c           |    2 +-
> >  2 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> > index bb7f309..93aca12 100644
> > --- a/include/linux/hardirq.h
> > +++ b/include/linux/hardirq.h
> > @@ -82,11 +82,15 @@
> >   * Are we in a softirq context? Interrupt context?
> >   * in_softirq - Are we currently processing softirq or have bh disabled?
> >   * in_serving_softirq - Are we currently processing softirq?
> > + * in_serving_interrupt - Are we currently processing softirq, nmi or
> > + *                        hardware interrupt?
> >   */
> >  #define in_irq()		(hardirq_count())
> >  #define in_softirq()		(softirq_count())
> >  #define in_interrupt()		(irq_count())
> >  #define in_serving_softirq()	(softirq_count() & SOFTIRQ_OFFSET)
> > +#define in_serving_interrupt()	(preempt_count() & (HARDIRQ_MASK \
> > +					 | SOFTIRQ_OFFSET | NMI_MASK))
> >  
> >  /*
> >   * Are we in NMI context?
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index 752d2c0..0c78ae6 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -896,7 +896,7 @@ void do_exit(long code)
> >  
> >  	WARN_ON(blk_needs_flush_plug(tsk));
> >  
> > -	if (unlikely(in_interrupt()))
> > +	if (unlikely(in_serving_interrupt()))
> >  		panic("Aiee, killing interrupt handler!");
> >  	if (unlikely(!tsk->pid))
> >  		panic("Attempted to kill the idle task!");
> > -- 
> > 1.7.7.6
> > 
> > 
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-- 
Regards,
Alexander Gordeev
agordeev@redhat.com