Re: Question about (or bug in?) the kobject implementation

[Greg KH <greg@kroah.com>]

On Wed, Feb 25, 2004 at 10:05:37AM -0500, Alan Stern wrote:
> Is it supposed to be legal to repeatedly call kobject_add() and 
> kobject_del() for the same kobject?  That is, is
> 
> 	kobject_add(&kobj);
> 	...
> 	kobject_del(&kobj);
> 	...
> 	kobject_add(&kobj);
> 	...
> 	kobject_del(&kobj);
> 
> supposed to work?

No.

> The API doesn't forbid it, and there's no apparent reason why it
> should be illegal.

We prevent race conditions in kobject_put() by saying "Don't do that!"
:)

Seriously, once kobject_del() is called, you can't safely call
kobject_get() anymore on that object.

If you can think of a way we can implement this in the code to prevent
people from doing this, please send a patch.  We've been getting by
without such a "safeguard" so far...

> Why would anyone want to do this, you ask?  Well the USB subsystem does it 
> already.  Each USB device can have several configurations, only one of 
> which is active at any time.  Corresponding to each configuration is a set 
> of struct devices, and they (together with their embedded kobjects) are 
> allocated and initialized when the USB device is first detected.  The 
> struct devices are add()'ed and del()'ed as configurations are activated 
> and deactivated, leading to just the sort of call sequence shown above.

Then we need to fix this.

thanks,

greg k-h