Re: atomic reference counting primatives.

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 05/21/04

  • Next message: John Baldwin: "Re: atomic reference counting primatives." 
    To: freebsd-arch@FreeBSD.org
Date: Fri, 21 May 2004 09:59:24 -0400

    On Thursday 20 May 2004 10:54 pm, M. Warner Losh wrote:
    > In message:
    > <Pine.BSF.4.21.0405201340590.72391-100000@InterJet.elischer.org>
    >
    > Julian Elischer <julian@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@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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