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Re: [Xen-devel] [PATCH v4 2/2] arm: proper ordering for correct execution of gic_update_one_lr and vgic_store_itargetsr

On 16/02/2017 22:10, Stefano Stabellini wrote:
On Thu, 16 Feb 2017, Julien Grall wrote:
Hi Stefano,

On 11/02/17 02:05, Stefano Stabellini wrote:
Concurrent execution of gic_update_one_lr and vgic_store_itargetsr can
result in the wrong pcpu being set as irq target, see

To solve the issue, add barriers, remove an irq from the inflight
queue, only after the affinity has been set. On the other end, write the
new vcpu target, before checking GIC_IRQ_GUEST_MIGRATING and inflight.

Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
 xen/arch/arm/gic.c     | 3 ++-
 xen/arch/arm/vgic-v2.c | 4 ++--
 xen/arch/arm/vgic-v3.c | 4 +++-
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
index a5348f2..bb52959 100644
--- a/xen/arch/arm/gic.c
+++ b/xen/arch/arm/gic.c
@@ -503,12 +503,13 @@ static void gic_update_one_lr(struct vcpu *v, int i)
              !test_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
             gic_raise_guest_irq(v, irq, p->priority);
         else {
-            list_del_init(&p->inflight);
             if ( test_and_clear_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
                 struct vcpu *v_target = vgic_get_target_vcpu(v, irq);
                 irq_set_affinity(p->desc, cpumask_of(v_target->processor));
+            smp_mb();
+            list_del_init(&p->inflight);

I don't understand why you remove from the inflight list afterwards. If you do
that you introduce that same problem as discussed in

As long as the interrupt is routed to the pCPU running gic_update_one_lr, the
interrupt cannot fired because the interrupts are masked.

This is not accurate: it is possible to receive a second interrupt
notification while the first one is still active.

Can you detail here? Because if you look at how gic_update_one_lr is called from gic_clear_lrs, interrupts are masked.

So it cannot be received by Xen while you are in gic_update_one_lr and before irq_set_affinity is called.

However, as soon as irq_set_affinity is called the interrupt may fire
on the other pCPU.

This is true.

However, list_del_init is not atomic and not protected by any lock. So
vgic_vcpu_inject_irq may see a corrupted version of {p,n}->inflight.

Did I miss anything?

Moving list_del_init later ensures that there are no conflicts between
gic_update_one_lr and vgic_store_itargetsr (more specifically,
vgic_migrate_irq). If you look at the implementation of
vgic_migrate_irq, all checks depends on list_empty(&p->inflight). If we
don't move list_del_init later, given that vgic_migrate_irq can be
called with a different vgic lock taken than gic_update_one_lr, the
following scenario can happen:

  CPU0: gic_update_one_lr               CPU1: vgic_store_itargetsr
  remove from inflight
  read rank->vcpu (intermediate)

It is only true if vgic_store_itargetsr is testing GIC_IRQ_GUEST_MIGRATING here and it was clear.

                                        set rank->vcpu (final)
                                          if (!inflight) irq_set_affinity 
  irq_set_affinity (intermediate)

As a result, the irq affinity is set to the wrong cpu. With this patch,
this problem doesn't occur.

However, you are right that both in the case of gic_update_one_lr and
vgic_migrate_irq, as well as the case of gic_update_one_lr and
vgic_vcpu_inject_irq that you mentioned, list_del_init (from
gic_update_one_lr) is potentially run as the same time as list_empty
(from vgic_migrate_irq or from vgic_vcpu_inject_irq), and they are not

Also see this other potential issue: 

All these concurrent accesses are difficult to understand and to deal
with. This is why my original suggestion was to use the old vcpu vgic
lock, rather then try to ensure safe concurrent accesses everywhere.
That option is still open and would solve both problems.
We only need to:

- store the vcpu to which an irq is currently injected
- check the new irq->vcpu field, and take the right vgic lock
something like http://marc.info/?l=xen-devel&m=148237295920492&w=2, but
would need improvements

Much simpler, right?

Would not it be easier to just take the desc->lock to protect the concurrent access?


Julien Grall

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