Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH] xen/events: Mask a moving irq | From | Boris Ostrovsky <> | Date | Thu, 17 Mar 2016 13:49:42 -0400 |
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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
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