Clear IRQ affinity when unregistered

When we offline a CPU, migrate_irqs() tries to determine whether the
affinity bits of the IRQ descriptor match any of the remaining online CPUs. If not, it fixes up the interrupt to point somewhere else. Unfortunately, if an IRQ is unregistered the IRQ descriptor may still have affinity to the CPU being offlined, but the no_irq_chip handler doesn't provide a set_affinity function. This causes us to hit the WARN_ON in migrate_irqs().

The easiest solution seems to be setting all the bits in the affinity
mask when the last interrupt is removed from the vector. I hit this on an older kernel with Xen/ia64 using driver domains (so it probably needs more testing on upstream). Xen essentially uses the bind/unbind interface in sysfs to unregister a device from a driver and thus unregister the interrupt. Thanks,

Alex

---

diff -r c73faa796cbf arch/ia64/kernel/iosapic.c
@@ -925,6 +925,11 @@ iosapic_unregister_intr (unsigned int gs
 			/* Clear the interrupt controller descriptor */
 			idesc->chip = &no_irq_type;
 
+#ifdef CONFIG_SMP
+			/* Clear affinity */
+			cpus_setall(idesc->affinity);
+#endif
+
 			/* Clear the interrupt information */
 			memset(&iosapic_intr_info[vector], 0,
 			       sizeof(struct iosapic_intr_info));