On Tue, 21 Feb 2017, Alan Stern wrote:
> 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.
>>
>
> 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?
Access of usb_class->kref is only from either init_usb_class()
or destroy_usb_class(), and both these functions are now protected
with Mutex Locking in patch v3, so there is no chance of race condition
as per above scenarios.
> 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.
As per my understanding, I found to be one of the best possible solution
for this problem and this solutiuon don't have any side effect.
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;
