Re: [RFC] Refcounting of objects part of a lockfree collection

[Dipankar Sarma <dipankar@in.ibm.com>]

On Wed, Jul 14, 2004 at 12:07:00AM -0700, Greg KH wrote:
> On Wed, Jul 14, 2004 at 10:23:50AM +0530, Ravikiran G Thirumalai wrote:
> > 
> > The attatched patch provides infrastructure for refcounting of objects
> > in a rcu protected collection.
> 
> This is really close to the kref implementation.  Why not just use that
> instead?

Well, the kref has the same get/put race if used in a lock-free
look-up. When you do a kref_get() it is assumed that another
cpu will not see a 1-to-0 transition of the reference count.
If that indeed happens, ->release() will get invoked more
than once for that object which is bad. Kiran's patch actually
solves this fundamental lock-free ref-counting problem.

The other issue is that there are many refcounted data structures
like dentry, dst_entry, file etc. that do not use kref. If everybody
were to use kref, we could possibly apply Kiran's lock-free extensions
to kref itself and be done with it. Until then, we need the lock-free
refcounting support from non-kref refcounting objects.

Thanks
Dipankar