[PATCH 1/2] drivers/hid: Eliminate a double lock

[PATCH 1/2] drivers/hid: Eliminate a double lock

[Julia Lawall]

From: Julia Lawall <julia@diku.dk>

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>

---
Compile tested only.

 drivers/hid/hid-debug.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
index c940267..850d02a 100644
--- 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);

Re: [PATCH 1/2] drivers/hid: Eliminate a double lock

[Jiri Kosina]

On Sun, 20 Jun 2010, Julia Lawall wrote:

> From: Julia Lawall <julia@diku.dk>
> 
> 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/)
[ ... ]
> diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
> index c940267..850d02a 100644
> --- 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);

Applied, thanks Julia.

-- 
Jiri Kosina
SUSE Labs, Novell Inc.