x86/irq: Use proper locking in check_irq_vectors_for_cpu_disable()

It's unsafe to examine fields in the irq descriptor w/o holding the
descriptor lock. Add proper locking.

While at it add a comment why the vector check can run lock less
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: xiao jin <jin.xiao@intel.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Link: http://lkml.kernel.org/r/20150705171102.236544164@linutronix.de

arch/x86/kernel/irq.c

@@ -347,14 +347,22 @@ int check_irq_vectors_for_cpu_disable(void)
 			if (!desc)
 				continue;
 
+			/*
+			 * Protect against concurrent action removal,
+			 * affinity changes etc.
+			 */
+			raw_spin_lock(&desc->lock);
 			data = irq_desc_get_irq_data(desc);
 			cpumask_copy(&affinity_new, data->affinity);
 			cpumask_clear_cpu(this_cpu, &affinity_new);
 
 			/* Do not count inactive or per-cpu irqs. */
-			if (!irq_has_action(irq) || irqd_is_per_cpu(data))
+			if (!irq_has_action(irq) || irqd_is_per_cpu(data)) {
+				raw_spin_unlock(&desc->lock);
 				continue;
+			}
 
+			raw_spin_unlock(&desc->lock);
 			/*
 			 * A single irq may be mapped to multiple
 			 * cpu's vector_irq[] (for example IOAPIC cluster
@@ -385,6 +393,9 @@ int check_irq_vectors_for_cpu_disable(void)
 		 * vector. If the vector is marked in the used vectors
 		 * bitmap or an irq is assigned to it, we don't count
 		 * it as available.
+		 *
+		 * As this is an inaccurate snapshot anyway, we can do
+		 * this w/o holding vector_lock.
 		 */
 		for (vector = FIRST_EXTERNAL_VECTOR;
 		     vector < first_system_vector; vector++) {