> On Thr, 2 Mar 2017, Ajay Kaher wrote:
>> On Wed, 1 Mar 2017, Alan Stern wrote:
>>> On Wed, 1 Mar 2017, Ajay Kaher wrote:
>>>> On Mon, 22 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.
>>
>> Under the circumstances, your patch is acceptable.
>>
>> If you really want to make the point crystal clear, you could replace
>> usb_class->kref with an ordinary integer counter. Then it would be
>> obvious that there are no references other than the ones taken by
>> init_usb_class() and released by destroy_usb_class().
>
> 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 :)
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;
