[PATCH 07/14] module: Fix invalid wakeup in wait_for_zero_refcount

[Libin]

If thread is preempted before calling set_current_state(TASK_INTERRUPTIBLE),
and the other thread set the condition followed with wake_up_process. After
that when this thread is re-scheduled, calling set_current_state to set itself
as state TASK_INTERRUPTIBLE, if it is preempted again after that and before
__set_current_state(TASK_RUNNING), it triggers the invalid wakeup problem.
-----------------------
wait_for_zero_refcount()
-----------------------
...
for (;;) {
	pr_debug("Looking at refcount...\n");
	set_current_state(TASK_UNINTERRUPTIBLE);
	if (module_refcount(mod) == 0)
		break;
	schedule();
}
__set_current_state(TASK_RUNNING);
...

To solve this problem, using preempt_disable() to bound the operaion that
setting the task state and the conditions(set by the wake thread) validation.
-----------------------
wait_for_zero_refcount()
-----------------------
...
preempt_disable();
for (;;) {
	pr_debug("Looking at refcount...\n");
	set_current_state(TASK_UNINTERRUPTIBLE);
	if (module_refcount(mod) == 0)
		break;
	preempt_enable();
	schedule();
	preempt_disable();
}
__set_current_state(TASK_RUNNING);
preempt_enable();
...

Signed-off-by: Libin <huawei.libin@xxxxxxxxxx>
---
 kernel/module.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/module.c b/kernel/module.c
index 2069158..22064e9 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -816,14 +816,18 @@ static void wait_for_zero_refcount(struct module *mod)
 {
 	/* Since we might sleep for some time, release the mutex first */
 	mutex_unlock(&module_mutex);
+	preempt_disable();
 	for (;;) {
 		pr_debug("Looking at refcount...\n");
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		if (module_refcount(mod) == 0)
 			break;
+		preempt_enable();
 		schedule();
+		preempt_disable();
 	}
-	current->state = TASK_RUNNING;
+	__set_current_state(TASK_RUNNING);
+	preempt_enable();
 	mutex_lock(&module_mutex);
 }
 
-- 
1.8.2.1


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