HID: eliminate a double lock in debug code

The path around the loop ends with the lock held, so the call to mutex_lock
is moved before the beginning of the loop.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@locked@
expression E1;
position p;
@@

read_lock(E1@p,...);

@r exists@
expression x <= locked.E1;
expression locked.E1;
expression E2;
identifier lock;
position locked.p,p1,p2;
@@

*lock@p1 (E1@p,...);
... when != E1
    when != \(x = E2\|&x\)
*lock@p2 (E1,...);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
index c940267..850d02a 100644 (file)
--- a/drivers/hid/hid-debug.c
+++ b/drivers/hid/hid-debug.c
@@ -949,8 +949,8 @@ static ssize_t hid_debug_events_read(struct file *file, char __user *buffer,
        int ret = 0, len;
        DECLARE_WAITQUEUE(wait, current);
 
+       mutex_lock(&list->read_mutex);
        while (ret == 0) {
-               mutex_lock(&list->read_mutex);
                if (list->head == list->tail) {
                        add_wait_queue(&list->hdev->debug_wait, &wait);
                        set_current_state(TASK_INTERRUPTIBLE);