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

[Jürgen Groß]

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().

> > 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.

> 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.

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.

> 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.


Juergen

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