atomic reference counting primatives.

[John Baldwin]

On Thursday 20 May 2004 10:54 pm, M. Warner Losh wrote:
> In message:
> <Pine.BSF.4.21.0405201340590.72391-100000 at InterJet.elischer.org>
>
>             Julian Elischer <julian at elischer.org> writes:
> : This has been raised before but I've come across uses for it again and
> : again so I'm raising it again.
> : JHB once posted some atomic referenc counting primatives. (Do you still
> : have them John?)
> : Alfred once said he had soem somewhere too, and other s have commentted
> : on this before, but we still don't seem to have any.
> :
> : every object is reference counted with its own code and
> : sometimes it's done poorly.
> :
> : Some peiople indicated that there are cases where a generic refcounter
> : can not be used and usd this as  a reason to not have one at all.
> :
> : So, here are some possibilities..
> : my first "write it down without too much thinking" effort..
> :
> : typedef {mumble} refcnt_t
> :
> : refcnt_add(refcnt_t *)
> :   Increments the reference count.. no magic except to be atomic.
> :
> :
> : int	refcnt_drop(refcnt *, struct mutex *)
> :  Decrements the refcount. If it goes to 0 it returns 0 and locks the
> : mutex  (if the mutex is supplied)..
>
> What prevents refcnt_add() from happening after ref count drops to 0?
> Wouldn't that be a race?  Eg, if we have two threads:
>
>
> 	Thread A			Thread B
>
> 	objp = lookup();
> [1]					refcnt_drop(&objp->ref, &objp->mtx);
> [2]	refcnt_add(&obj->ref);
> 					BANG!
>
> If [1] happens before [2], then bad things happen at BANG!  If [2]
> happens before [1], then the mutex won't be locked at BANG and things
> is good.  Thread A believes it has a valid reference to objp after the
> refcnt_add and no way of knowing otherwise.
>
> Is there a safe way to use the API into what you are proposing?

This situation can't happen if you are properly using reference counting.  For 
the reference count to be at 1 in thread B, it has to have the only reference 
meaning that the object has already been removed from any lists, etc.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org