Date: Fri, 21 May 2004 09:59:24 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-arch@FreeBSD.org Cc: julian@elischer.org Subject: Re: atomic reference counting primatives. Message-ID: <200405210959.25368.jhb@FreeBSD.org> In-Reply-To: <20040520.205403.08940889.imp@bsdimp.com> References: <Pine.BSF.4.21.0405201340590.72391-100000@InterJet.elischer.org> <20040520.205403.08940889.imp@bsdimp.com>
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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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