[patch 2/2] hiddev: fix waitqueue usage

[Johannes Weiner]

DECLARE_WAITQUEUE doesn't initialize the wait descriptor's task_list
to 'empty' but to zero.

prepare_to_wait() will not enqueue the descriptor to the waitqueue and
finish_wait() will do list_del_init() on a list head that contains
NULL pointers, which oopses.

This was introduced by 079034073 "HID: hiddev cleanup -- handle all
error conditions properly".

The prior code used an unconditional add_to_waitqueue() which didn't
care about the wait descriptor's list head and enqueued the thing
unconditionally.

The new code uses prepare_to_wait() which DOES check the prior list
state, so use DEFINE_WAIT instead.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Oliver Neukum <oliver@xxxxxxxxxxx>
---
 drivers/hid/usbhid/hiddev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -323,7 +323,7 @@ static ssize_t hiddev_write(struct file 
  */
 static ssize_t hiddev_read(struct file * file, char __user * buffer, size_t count, loff_t *ppos)
 {
-	DECLARE_WAITQUEUE(wait, current);
+	DEFINE_WAIT(wait);
 	struct hiddev_list *list = file->private_data;
 	int event_size;
 	int retval;


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