stop_machine: fix error code handling on multiple cpus

Using |= for updating a value which might be updated on several cpus
concurrently will not always work since we need to make sure that the
update happens atomically.
To fix this just use a write if the called function returns an error
code on a cpu. We end up writing the error code of an arbitrary cpu
if multiple ones fail but that should be sufficient.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/stop_machine.c

@@ -66,6 +66,7 @@ static void stop_cpu(struct work_struct *unused)
 	enum stopmachine_state curstate = STOPMACHINE_NONE;
 	struct stop_machine_data *smdata = &idle;
 	int cpu = smp_processor_id();
+	int err;
 
 	if (!active_cpus) {
 		if (cpu == first_cpu(cpu_online_map))
@@ -86,9 +87,11 @@ static void stop_cpu(struct work_struct *unused)
 				hard_irq_disable();
 				break;
 			case STOPMACHINE_RUN:
-				/* |= allows error detection if functions on
+				/* On multiple CPUs only a single error code
-				 * multiple CPUs. */
+				 * is needed to tell that something failed. */
-				smdata->fnret |= smdata->fn(smdata->data);
+				err = smdata->fn(smdata->data);
+				if (err)
+					smdata->fnret = err;
 				break;
 			default:
 				break;