freezer: fix current->state restoration race in refrigerator()
refrigerator() saves current->state before entering frozen state and
restores it before returning using __set_current_state(); however,
this is racy, for example, please consider the following sequence.
set_current_state(TASK_INTERRUPTIBLE);
try_to_freeze();
if (kthread_should_stop())
break;
schedule();
If kthread_stop() races with ->state restoration, the restoration can
restore ->state to TASK_INTERRUPTIBLE after kthread_stop() sets it to
TASK_RUNNING but kthread_should_stop() may still see zero
->should_stop because there's no memory barrier between restoring
TASK_INTERRUPTIBLE and kthread_should_stop() test.
This isn't restricted to kthread_should_stop(). current->state is
often used in memory barrier based synchronization and silently
restoring it w/o mb breaks them.
Use set_current_state() instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/freezer.c b/kernel/freezer.c
index 7be56c5..3f46010 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -58,7 +58,13 @@
current->flags &= ~PF_FREEZING;
pr_debug("%s left refrigerator\n", current->comm);
- __set_current_state(save);
+
+ /*
+ * Restore saved task state before returning. The mb'd version
+ * needs to be used; otherwise, it might silently break
+ * synchronization which depends on ordered task state change.
+ */
+ set_current_state(save);
}
EXPORT_SYMBOL(refrigerator);
