Date: Tue, 7 Mar 2017 10:00:29 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Ajay Kaher <ajay.kaher@samsung.com>
cc: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        AMAN DEEP <aman.deep@samsung.com>,
        HEMANSHU SRIVASTAVA <hemanshu.s@samsung.com>
Subject: Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race
 Condition when two USB class drivers try to call init_usb_class simultaneously
In-Reply-To: <20170307043537epcms5p7e4fcfc351c4f193e454c9ea2e271e562@epcms5p7>
Message-ID: <Pine.LNX.4.44L0.1703070959400.1875-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 3019
Lines: 83

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@rowland.harvard.edu>

Alan Stern