Re: [Xen-devel] [PATCH] xen/events: Mask a moving irq

[Boris Ostrovsky]

On 03/17/2016 01:29 PM, David Vrabel wrote:
> On 17/03/16 16:53, Boris Ostrovsky wrote:
> > On 03/17/2016 12:03 PM, David Vrabel wrote:
> > > On 17/03/16 12:45, Boris Ostrovsky wrote:
> > > > Moving an unmasked irq may result in irq handler being invoked on both
> > > > source and target CPUs.
> > > >
> > > > With 2-level this can happen as follows:
> > > >
> > > > On source CPU:
> > > >           evtchn_2l_handle_events() ->
> > > >               generic_handle_irq() ->
> > > >                   handle_edge_irq() ->
> > > >                      eoi_pirq():
> > > >                          irq_move_irq(data);
> > > >
> > > >                          /***** WE ARE HERE *****/
> > > >
> > > >                          if (VALID_EVTCHN(evtchn))
> > > >                              clear_evtchn(evtchn);
> > > >
> > > > If at this moment target processor is handling an unrelated event in
> > > > evtchn_2l_handle_events()'s loop it may pick up our event since target's
> > > > cpu_evtchn_mask claims that this event belongs to it *and* the event is
> > > > unmasked and still pending. At the same time, source CPU will continue
> > > > executing its own handle_edge_irq().
> > > >
> > > > With FIFO interrupt the scenario is similar: irq_move_irq() may result
> > > > in a EVTCHNOP_unmask hypercall which, in turn, may make the event
> > > > pending on the target CPU.
> > > >
> > > > We can avoid this situation by moving and clearing the event while
> > > > keeping event masked.
> > > Can you do:
> > >
> > >      if (unlikely(irqd_is_setaffinity_pending(data))) {
> > >          masked = test_and_set_mask()
> > >
> > >          clear_evtchn()
> > >          irq_move_masked_irq()
> > I did think about this but then I wasn't sure whether this might open
> > some other window for things to sneak in. It shouldn't but these things
> > are rather subtle so I'd rather leave the order of how operations are
> > done unchanged.
> This is the order your patch has though.  I'm confused.

Ugh, sorry --- I misread what you wrote, I thought you wanted to clear 
before masking. Which wouldn't make any sense.

So yes, what you are suggesting is better.

-borsi