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Re: [Xen-devel] [PATCH] arm/acpi: Fix the deadlock in function vgic_lock_rank()

(CC Wei Liu)

Hi Stefano,

On 30/05/2016 14:16, Stefano Stabellini wrote:
On Fri, 27 May 2016, Julien Grall wrote:
Hello Shanker,

On 27/05/16 01:39, Shanker Donthineni wrote:
Commit 9d77b3c01d1261c (Configure SPI interrupt type and route to
Dom0 dynamically) causing dead loop inside the spinlock function.
Note that spinlocks in XEN are not recursive. Re-acquiring a spinlock
that has already held by calling CPU leads to deadlock. This happens
whenever dom0 does writes to GICD regs ISENABLER/ICENABLER.

Thank you for spotting it, I have not noticed it while I was  reviewing, only
tested on a model without any SPIs.

The following call trace explains the problem.

     vgic_lock_rank()  -->  acquiring first time
               vgic_lock_rank()  -->  attemping acquired lock

The simple fix release spinlock before calling vgic_enable_irqs()
and vgic_disable_irqs().

You should explain why you think it is valid to release the lock earlier.

In this case, I think the fix is not correct because the lock is protecting
both the register value and the internal state in Xen (modified by
vgic_enable_irqs). By releasing the lock earlier, they may become inconsistent
if another vCPU is disabling the IRQs at the same time.

I agree, the vgic_enable_irqs call need to stay within the
vgic_lock_rank/vgic_unlock_rank region.

I cannot find an easy fix which does not involve release the lock. When I was
reviewing this patch, I suggested to split the IRQ configuration from the

Yes, the routing doesn't need to be done from vgic_enable_irqs. It is
not nice. That would be the ideal fix, but it is not trivial.

For 4.7 we could consider reverting 9d77b3c01d1261c. The only other
thing that I can come up with which is simple would be improving
gic_route_irq_to_guest to cope with callers that have the vgic rank lock
already held (see below, untested) but it's pretty ugly.

We are close to release Xen 4.7, so I think we should avoid to touch the common interrupt code (i.e not only used by ACPI).

ACPI can only be enabled in expert mode and will be a tech-preview for Xen 4.7. So I would revert the patch. SPIs will not be routed, but it is better than a deadlock.

I would also replace the patch with a warning until the issue will be fixed in Xen 4.8.

Any opinions?

+int gic_route_irq_to_guest(struct domain *d, unsigned int virq,
+                           struct irq_desc *desc, unsigned int priority)
+    unsigned long flags;
+    int lock = 0, retval;
+    struct vgic_irq_rank *rank;
+    /* Use vcpu0 to retrieve the pending_irq struct. Given that we only
+     * route SPIs to guests, it doesn't make any difference. */
+    rank = vgic_rank_irq(d->vcpu[0], virq);
+    /* Take the rank spinlock unless it has already been taken by the
+     * caller. */
+    if ( !spin_is_locked(&rank->lock) ) {

AFAICT, spin_is_locked only tell us that someone has locked the rank. So this would be unsafe.


Julien Grall

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