Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Tue, Mar 07, 2017 at 04:35:37AM +0000, 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.
> 
> > 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
>  

Can you resend this in a format that I can apply it in?  I suggest
reading Documentation/SubmittingPatches.  If you have any questions
about the correct format, please let me know.

Also add Alan's ack to it.

thanks,

greg k-h