Messages in this thread | | | Subject | Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids | From | Jürgen Groß <> | Date | Mon, 8 Feb 2021 11:22:52 +0100 |
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On 08.02.21 10:54, Julien Grall wrote: > > > On 08/02/2021 09:41, Jürgen Groß wrote: >> On 08.02.21 10:11, Julien Grall wrote: >>> Hi Juergen, >>> >>> On 07/02/2021 12:58, Jürgen Groß wrote: >>>> On 06.02.21 19:46, Julien Grall wrote: >>>>> Hi Juergen, >>>>> >>>>> On 06/02/2021 10:49, Juergen Gross wrote: >>>>>> The first three patches are fixes for XSA-332. The avoid WARN splats >>>>>> and a performance issue with interdomain events. >>>>> >>>>> Thanks for helping to figure out the problem. Unfortunately, I >>>>> still see reliably the WARN splat with the latest Linux master >>>>> (1e0d27fce010) + your first 3 patches. >>>>> >>>>> I am using Xen 4.11 (1c7d984645f9) and dom0 is forced to use the 2L >>>>> events ABI. >>>>> >>>>> After some debugging, I think I have an idea what's went wrong. The >>>>> problem happens when the event is initially bound from vCPU0 to a >>>>> different vCPU. >>>>> >>>>> From the comment in xen_rebind_evtchn_to_cpu(), we are masking the >>>>> event to prevent it being delivered on an unexpected vCPU. However, >>>>> I believe the following can happen: >>>>> >>>>> vCPU0 | vCPU1 >>>>> | >>>>> | Call xen_rebind_evtchn_to_cpu() >>>>> receive event X | >>>>> | mask event X >>>>> | bind to vCPU1 >>>>> <vCPU descheduled> | unmask event X >>>>> | >>>>> | receive event X >>>>> | >>>>> | handle_edge_irq(X) >>>>> handle_edge_irq(X) | -> handle_irq_event() >>>>> | -> set IRQD_IN_PROGRESS >>>>> -> set IRQS_PENDING | >>>>> | -> evtchn_interrupt() >>>>> | -> clear IRQD_IN_PROGRESS >>>>> | -> IRQS_PENDING is set >>>>> | -> handle_irq_event() >>>>> | -> evtchn_interrupt() >>>>> | -> WARN() >>>>> | >>>>> >>>>> All the lateeoi handlers expect a ONESHOT semantic and >>>>> evtchn_interrupt() is doesn't tolerate any deviation. >>>>> >>>>> I think the problem was introduced by 7f874a0447a9 ("xen/events: >>>>> fix lateeoi irq acknowledgment") because the interrupt was disabled >>>>> previously. Therefore we wouldn't do another iteration in >>>>> handle_edge_irq(). >>>> >>>> I think you picked the wrong commit for blaming, as this is just >>>> the last patch of the three patches you were testing. >>> >>> I actually found the right commit for blaming but I copied the >>> information from the wrong shell :/. The bug was introduced by: >>> >>> c44b849cee8c ("xen/events: switch user event channels to lateeoi model") >>> >>>> >>>>> Aside the handlers, I think it may impact the defer EOI mitigation >>>>> because in theory if a 3rd vCPU is joining the party (let say vCPU >>>>> A migrate the event from vCPU B to vCPU C). So info->{eoi_cpu, >>>>> irq_epoch, eoi_time} could possibly get mangled? >>>>> >>>>> For a fix, we may want to consider to hold evtchn_rwlock with the >>>>> write permission. Although, I am not 100% sure this is going to >>>>> prevent everything. >>>> >>>> It will make things worse, as it would violate the locking hierarchy >>>> (xen_rebind_evtchn_to_cpu() is called with the IRQ-desc lock held). >>> >>> Ah, right. >>> >>>> >>>> On a first glance I think we'll need a 3rd masking state ("temporarily >>>> masked") in the second patch in order to avoid a race with lateeoi. >>>> >>>> In order to avoid the race you outlined above we need an "event is >>>> being >>>> handled" indicator checked via test_and_set() semantics in >>>> handle_irq_for_port() and reset only when calling clear_evtchn(). >>> >>> It feels like we are trying to workaround the IRQ flow we are using >>> (i.e. handle_edge_irq()). >> >> I'm not really sure this is the main problem here. According to your >> analysis the main problem is occurring when handling the event, not when >> handling the IRQ: the event is being received on two vcpus. > > I don't think we can easily divide the two because we rely on the IRQ > framework to handle the lifecycle of the event. So... > >> >> Our problem isn't due to the IRQ still being pending, but due it being >> raised again, which should happen for a one shot IRQ the same way. > > ... I don't really see how the difference matter here. The idea is to > re-use what's already existing rather than trying to re-invent the wheel > with an extra lock (or whatever we can come up).
The difference is that the race is occurring _before_ any IRQ is involved. So I don't see how modification of IRQ handling would help.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |