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

Boris Ostrovsky
2016-03-17T17:49:40Z

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