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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.

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.)

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