linux-kernel - RE: Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb

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Message-id: <20170222125736epcms5p1eed1cc9c1db3b402a7f7c46df3e04f1a@epcms5p1>
Date:   Wed, 22 Feb 2017 12:57:36 +0000
From:   Ajay Kaher <ajay.kaher@...sung.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        AMAN DEEP <aman.deep@...sung.com>,
        HEMANSHU SRIVASTAVA <hemanshu.s@...sung.com>
Subject: RE: Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race
 Condition when two USB class drivers try to call init_usb_class simultaneously

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;