Re: [Xen-devel] [RFC PATCH 01/10] ARM: vGIC: remove rank lock from IRQ routing functions

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 08/05/2017 22:53, Stefano Stabellini wrote:
> On Mon, 8 May 2017, Julien Grall wrote:
> > Hi Andre,
> >
> > On 08/05/17 10:15, Andre Przywara wrote:
> > > On 04/05/17 16:53, Julien Grall wrote:
> > > > Hi Andre,
> > > >
> > > > On 04/05/17 16:31, Andre Przywara wrote:
> > > > > gic_route_irq_to_guest() and gic_remove_irq_from_guest() take the rank
> > > > > lock, however never actually access the rank structure.
> > > > > Avoid taking the lock in those two functions and remove some more
> > > > > unneeded code on the way.
> > > > The rank is here to protect p->desc when checking that the virtual
> > > > interrupt was not yet routed to another physical interrupt.
> > > Really? To me that sounds quite surprising.
> > > My understanding is that the VGIC VCPU lock protected the pending_irq
> > > (and thus the desc pointer?) so far, and the desc itself has its own lock.
> > > According to the comment in the struct irq_rank declaration the lock
> > > protects the members of this struct only.
> > >
> > > Looking briefly at users of pending_irq->desc (for instance
> > > gicv[23]_update_lr() or gic_update_one_lr()) I can't see any hint that
> > > they care about the lock.
> > >
> > > So should that be fixed or at least documented?
> > My understanding is this rank lock is preventing race between two updates of
> > p->desc. This can happen if gic_route_irq_to_guest(...) is called concurrently
> > with the same vIRQ but different pIRQ.
> >
> > If you drop this lock, nothing will protect that anymore.
> The desc->lock in route_irq_to_guest is protecting against concurrent
> changes to the same physical irq.
>
> The vgic_lock_rank in gic_route_irq_to_guest is protecting against
> concurrent changes to the same virtual irq.
>
> In other words, the current code does:
> 1) lock physical irq
> 2) lock virtual irq
> 3) establish mapping virtual irq - physical irq
>
> Andre, sorry for not seeing this earlier.
>
>
> > > > Without this locking, you can have two concurrent call of
> > > > gic_route_irq_to_guest that will update the same virtual interrupt but
> > > > with different physical interrupts.
> > > >
> > > > You would have to replace the rank lock by the per-pending_irq lock to
> > > > keep the code safe.
> > > That indeed sounds reasonable.
> > As you mentioned IRL, the current code may lead to a deadlock due to locking
> > order.
> >
> > Indeed routing an IRQ (route_irq_to_guest) will take:
> >         1) desc lock (in route_irq_to_guest)
> >         2) rank lock (in gic_route_irq_to_guest)
> >
> > Whilst the MMIO emulation of ISENABLER_* will take:
> >         1) rank lock
> >         2) desc lock (in vgic_enable_irqs)
> Yes, you are right, but I don't think that is an issue because
> route_irq_to_guest is expected to be called before the domain is
> started, which is consistent with what you wrote below that routing
> should be done *before* running the VM.
I didn't consider as a big issue because of that and the fact it 
currently can only happen via a domctl or during dom0 building.
> To be clear: I am not arguing for keeping the code as-is, just trying to
> understand it.
>
>
> > Using the per-pending_irq lock will not solve the deadlock. I though a bit
> > more to the code. I believe the routing of SPIs/PPIs after domain creation
> > time is a call to mistake and locking nightmare. Similarly an interrupt should
> > stay routed for the duration of the domain life.
> It looks like current routing code was already written with that assumption.
>
>
> > So I would forbid IRQ routing after domain creation (see d->creation_finished)
> > and remove IRQ whilst routing (I think with d->is_dying). This would have an
> > race between the routing and the rest of the vGIC code.
> I am fine with that.
>
>
> > However, this would not prevent the routing function to race against itself.
> > For that I would take the vgic domain lock, it is fine because routing is not
> > something we expect to happen often.
> >
> > Any opinions?
> The changes done by gic_route_irq_to_guest are specific to one virtual
> irq and one physical irq. In fact, they establish the mapping between
> the two.
>
> Given that concurrent changes to a physical irq are protected by the
> desc->lock in route_irq_to_guest, why do you think that taking the new
> pending_irq lock in gic_route_irq_to_guest would not be sufficient?
> (gic_route_irq_to_guest is always called by route_irq_to_guest.)
Because if we just replace the rank lock by the pending lock there 
deadlock would still be there. I guess we can take the pending_irq lock 
in route_irq_to_guest before the desc lock.
But I like the idea of hiding the vgic locking in gic_route_irq_to_guest 
to keep irq.c fairly Interrupt Controller agnostic.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel