Re: [Xen-devel] Re: [PATCH 09/14] xen/pvticketlock: Xen implementation for PV ticket locks

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 11/17/2010 02:34 AM, Jan Beulich wrote:
>> Actually, on second thoughts, maybe it doesn't matter so much.  The main
>> issue is making sure that the interrupt will make the VCPU drop out of
>> xen_poll_irq() - if it happens before xen_poll_irq(), it should leave
>> the event pending, which will cause the poll to return immediately.  I
>> hope.  Certainly disabling interrupts for some of the function will make
>> it easier to analyze with respect to interrupt nesting.
> That's not my main concern. Instead, what if you get interrupted
> anywhere here, the interrupt handler tries to acquire another
> spinlock and also has to go into the slow path? It'll overwrite part
> or all of the outer context's state.

That doesn't matter if the outer context doesn't end up blocking.  If it
has already blocked then it will unblock as a result of the interrupt;
if it hasn't yet blocked, then the inner context will leave the event
pending and cause it to not block.  Either way, it no longer uses or
needs that per-cpu state: it will return to the spin loop and (maybe)
get re-entered, setting it all up again.

I think there is a problem with the code as posted because it sets up
the percpu data before clearing the pending event, so it can end up
blocking with bad percpu data.

>> Another issue may be making sure the writes and reads of "w->want" and
>> "w->lock" are ordered properly to make sure that xen_unlock_kick() never
>> sees an inconsistent view of the (lock,want) tuple.  The risk being that
>> xen_unlock_kick() sees a random, spurious (lock,want) pairing and sends
>> the kick event to the wrong VCPU, leaving the deserving one hung.
> Yes, proper operation sequence (and barriers) is certainly
> required here. If you allowed nesting, this may even become
> simpler (as you'd have a single write making visible the new
> "head" pointer, after having written all relevant fields of the
> new "head" structure).

Yes, simple nesting should be quite straightforward (ie allowing an
interrupt handler to take some other lock than the one the outer context
is waiting on).

    J