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

[Julien Grall]

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().

> 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?

Note that are are other IRQ flows existing. We should have a look at 
them before trying to fix thing ourself.

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".

Cheers,

--
Julien Grall