stop_machine: fix race with return value

We should not access active.fnret outside the lock; in theory the next
stop_machine could overwrite it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 kernel/stop_machine.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff -r d7c9a15da615 kernel/stop_machine.c
--- a/kernel/stop_machine.c	Mon Nov 10 09:47:45 2008 +1100
+++ b/kernel/stop_machine.c	Tue Nov 11 23:19:47 2008 +1030
@@ -112,7 +112,7 @@
 int __stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)
 {
 	struct work_struct *sm_work;
-	int i;
+	int i, ret;
 
 	/* Set up initial state. */
 	mutex_lock(&lock);
@@ -137,8 +137,9 @@
 	/* This will release the thread on our CPU. */
 	put_cpu();
 	flush_workqueue(stop_machine_wq);
+	ret = active.fnret;
 	mutex_unlock(&lock);
-	return active.fnret;
+	return ret;
 }
 
 int stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)