FW: 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

[Ajay Kaher <ajay.kaher@xxxxxxxxxxx>]

> On Mon, 22 Feb 2017, Ajay Kaher 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.

Alan, I had shared modified Patch v3 as per your inputs to prevent
the race condition during simultaneously calling of init_usb_class().
If you think there is scope to improve the patch, please share your inputs.

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;