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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html