> 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().
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;
Thanks and Regards,
Ajay Kaher
