Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously

[Alan Stern]

On Mon, 20 Feb 2017, Ajay Kaher wrote:

> Alan, as per my understanding I have shifted the lock from
> release_usb_class() to destroy_usb_class() in patch v3. 
> If it is not right, please explain in detail which race condition
> I have missed and also share your suggestions.
> 
> thanks,
> ajay kaher
> 
> Signed-off-by: Ajay Kaher
> 
> ---
> 
>  drivers/usb/core/file.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
> index 822ced9..a12d184 100644
> --- a/drivers/usb/core/file.c
> +++ b/drivers/usb/core/file.c
> @@ -27,6 +27,7 @@
>  #define MAX_USB_MINORS 256
>  static const struct file_operations *usb_minors[MAX_USB_MINORS];
>  static DECLARE_RWSEM(minor_rwsem);
> +static DEFINE_MUTEX(init_usb_class_mutex);
> 
>  static int usb_open(struct inode *inode, struct file *file)
>  {
> @@ -109,8 +110,10 @@ static void release_usb_class(struct kref *kref)
> 
>  static void destroy_usb_class(void)
>  {
> +       mutex_lock(&init_usb_class_mutex);
>         if (usb_class)
>                 kref_put(&usb_class->kref, release_usb_class);
> +       mutex_unlock(&init_usb_class_mutex);
>  }
> 
>  int usb_major_init(void)
> @@ -171,7 +174,10 @@ int usb_register_dev(struct usb_interface *intf,
>         if (intf->minor >= 0)
>                 return -EADDRINUSE;
> 
> +       mutex_lock(&init_usb_class_mutex);
>         retval = init_usb_class();
> +       mutex_unlock(&init_usb_class_mutex);
> +
>         if (retval)
>                 return retval;

Have you considered what would happen if destroy_usb_class() ran, but 
some other CPU was still holding a reference to usb_class?  And what if 
the last reference gets dropped later on, while init_usb_class() is 
running?

Maybe that's not possible here, but it is possible in general for 
refcounted objects.  So yes, this code is probably okay, but it isn't 
good form.

Alan Stern