Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

[Jürgen Groß <jgross@xxxxxxxx>]

On 08.02.21 15:20, Julien Grall wrote:
> Hi Juergen,
>
> On 08/02/2021 13:58, Jürgen Groß wrote:
> > On 08.02.21 14:09, Julien Grall wrote:
> > > Hi Juergen,
> > >
> > > On 08/02/2021 12:31, Jürgen Groß wrote:
> > > > On 08.02.21 13:16, Julien Grall wrote:
> > > > > On 08/02/2021 12:14, Jürgen Groß wrote:
> > > > > > On 08.02.21 11:40, Julien Grall wrote:
> > > > > > > Hi Juergen,
> > > > > > >
> > > > > > > On 08/02/2021 10:22, Jürgen Groß wrote:
> > > > > > > > On 08.02.21 10:54, Julien Grall wrote:
> > > > > > > > > ... 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.
> > > > > > >
> > > > > > > Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:
> > > > > > >
> > > > > > > if ( irq in progress )
> > > > > > > {
> > > > > > >    set IRQS_PENDING
> > > > > > >    return;
> > > > > > > }
> > > > > > >
> > > > > > > do
> > > > > > > {
> > > > > > >    clear IRQS_PENDING
> > > > > > >    handle_irq()
> > > > > > > } while (IRQS_PENDING is set)
> > > > > > >
> > > > > > > IRQ handling flow like handle_fasteoi_irq() looks like:
> > > > > > >
> > > > > > > if ( irq in progress )
> > > > > > >    return;
> > > > > > >
> > > > > > > handle_irq()
> > > > > > >
> > > > > > > The latter flow would catch "spurious" interrupt and ignore them. 
> > > > > > > So it would handle nicely the race when changing the event affinity.
> > > > > >
> > > > > > Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
> > > > > > issued, thus having the same problem again? 
> > > > >
> > > > > Sorry I can't parse this.
> > > >
> > > > handle_fasteoi_irq() will do nothing "if ( irq in progress )". When is
> > > > this condition being reset again in order to be able to process another
> > > > IRQ?
> > > It is reset after the handler has been called. See handle_irq_event().
> >
> > Right. And for us this is too early, as we want the next IRQ being
> > handled only after we have called xen_irq_lateeoi().
>
> It is not really the next IRQ here. It is more a spurious IRQ because we 
> don't clear & mask the event right away. Instead, it is done later in 
> the handling.
>
> > > > I believe this will be the case before our "lateeoi" handling is
> > > > becoming active (more precise: when our IRQ handler is returning to
> > > > handle_fasteoi_irq()), resulting in the possibility of the same race we
> > > > are experiencing now.
> > >
> > > I am a bit confused what you mean by "lateeoi" handling is becoming 
> > > active. Can you clarify?
> >
> > See above: the next call of the handler should be allowed only after
> > xen_irq_lateeoi() for the IRQ has been called.
> >
> > If the handler is being called earlier we have the race resulting
> > in the WARN() splats.
>
> I feel it is dislike to understand race with just words. Can you provide 
> a scenario (similar to the one I originally provided) with two vCPUs and 
> show how this 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_fasteoi_irq(X)
                     |  -> handle_irq_event()
                     |   -> set IRQD_IN_PROGRESS
                     |   -> evtchn_interrupt()
                     |      -> evtchn->enabled = false
                     |   -> clear IRQD_IN_PROGRESS
handle_fasteoi_irq(X)|
-> evtchn_interrupt()|
   -> WARN()         |
                     | xen_irq_lateeoi(X)

> > > Note that are are other IRQ flows existing. We should have a look at 
> > > them before trying to fix thing ourself.
> >
> > Fine with me, but it either needs to fit all use cases (interdomain,
> > IPI, real interrupts) or we need to have a per-type IRQ flow.
>
> AFAICT, we already used different flow based on the use cases. Before 
> 2011, we used to use the fasteoi one but this was changed by the 
> following commit:

Yes, I know that.

> > I think we should fix the issue locally first, then we can start to do
> > a thorough rework planning. Its not as if the needed changes with the
> > current flow would be so huge, and I'd really like to have a solution
> > rather sooner than later. Changing the IRQ flow might have other side
> > effects which need to be excluded by thorough testing.
> I agree that we need a solution ASAP. But I am a bit worry to:
>    1) Add another lock in that event handling path.

Regarding complexity: it is very simple (just around masking/unmasking
of the event channel). Contention is very unlikely.

>    2) Add more complexity in the event handling (it is already fairly 
> difficult to reason about the locking/race)
>
> Let see what the local fix look like.

Yes.

> > > Although, the other issue I can see so far is handle_irq_for_port() 
> > > will update info->{eoi_cpu, irq_epoch, eoi_time} without any locking. 
> > > But it is not clear this is what you mean by "becoming active".
> >
> > As long as a single event can't be handled on multiple cpus at the same
> > time, there is no locking needed.
>
> Well, it can happen in the current code (see my original scenario). If 
> your idea fix it then fine.

I hope so.


Juergen

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