On Tue, 7 Mar 2017, Ajay Kaher wrote:
> > On Fri, 3 Mar 2017, Ajay Kaher wrote:
> >
> > > > usb_class->kref is not accessible outside the file.c
> > > > as usb_class is _static_ inside the file.c and
> > > > pointer of usb_class->kref is not passed anywhere.
> > > >
> > > > Hence as you wanted, there are no references of usb_class->kref
> > > > other than taken by init_usb_class() and released by destroy_usb_class().
> > >
> > > Verified the code again, I hope my last comments clarifed the things
> > > which came in your mind and helps you to accept the patch :)
> >
> > Your main point is that usb_class->kref is accessed from only two
> > points, both of which are protected by the new mutex. This means there
> > is no reason for the value to be a struct kref at all. You should
> > change it to an int (and change its name). Leaving it as a kref will
> > make readers wonder why it needs to be updated atomically.
>
> At many places in Linux kernel, instances of Kref have been used within
> Mutex, SpinLock and don’t have any side effect.
>
> Making to int and handle (i.e. get/put) it within file.c seems
> not good as we have Kref. Instead, we can have non_atomic version of kref.
> We can discuss about non_atomic kref in another thread, if you are interested.
Okay.
> > Also, why does destroy_usb_class() have that "if (usb_class) "test?
> > Isn't it true that usb_class can never be NULL there?
>
> Removed in Patch v4.
>
> thanks,
> ajay kaher
>
>
> Signed-off-by: Ajay Kaher
>
> ---
>
> drivers/usb/core/file.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
> index 822ced9..422ce7b 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 +110,9 @@ static void release_usb_class(struct kref *kref)
>
> static void destroy_usb_class(void)
> {
> - if (usb_class)
> - kref_put(&usb_class->kref, release_usb_class);
> + mutex_lock(&init_usb_class_mutex);
> + kref_put(&usb_class->kref, release_usb_class);
> + mutex_unlock(&init_usb_class_mutex);
> }
>
> int usb_major_init(void)
> @@ -171,7 +173,10 @@ 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;
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Alan Stern
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