genirq: Dont allow affinity mask to be updated on IPIs

The IPI domain re-purposes the IRQ affinity to signify the mask of CPUs
that this IPI will deliver to. This must not be modified before the IPI
is destroyed again, so set the IRQ_NO_BALANCING flag to prevent the
affinity being overwritten by setup_affinity().

Without this, if an IPI is reserved for a single target CPU, then
allocated using __setup_irq(), the affinity is overwritten with
cpu_online_mask. When ipi_destroy() is subsequently called on a
multi-cpu system, it will attempt to free cpumask_weight() IRQs
that were never allocated, and crash.

Fixes: d17bf24e69 ("genirq: Add a new generic IPI reservation code to irq core")
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Cc: ralf@linux-mips.org
Cc: Qais Yousef <qsyousef@gmail.com>
Cc: lisa.parratt@imgtec.com
Link: http://lkml.kernel.org/r/1461229712-13057-1-git-send-email-matt.redfearn@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/ipi.c

@@ -94,6 +94,7 @@ unsigned int irq_reserve_ipi(struct irq_domain *domain,
 		data = irq_get_irq_data(virq + i);
 		cpumask_copy(data->common->affinity, dest);
 		data->common->ipi_offset = offset;
+		irq_set_status_flags(virq + i, IRQ_NO_BALANCING);
 	}
 	return virq;