Re: about reference counter

[Greg KH <greg@xxxxxxxxx>]

On Fri, Oct 01, 2004 at 05:45:56PM -0400, lml@xxxxxxxxxxxxxxxx wrote:
> > > After that, they canread/write the object at the same time, right? It
> > > sounds like only the reference counter is atomic, while the content of
> > > the object is not protected. Please correct me if I am wrong.
> > 
> > Yes, they could both use the object at the same time.  But if you need
> > to protect from that, you need a lock, not only a reference count.
> 
> So what is the real purpose of using reference counter? It cannot
> protect other members in the object.

You are correct, that is not what a reference counter is for.

> Looks like it is only used to check if the object needed to be freed
> (when the counter is 0)? right?

Yes.

> Can you give an example?

struct urb.  When the last user of it is done, it is automatically
cleaned up and freed.  struct device, struct kobject, struct
class_device, struct class are all other examples.  The kernel is full
of them.

Hope this helps.

greg k-h

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