Date: Thu, 20 May 2004 20:54:03 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: julian@elischer.org Cc: arch@freebsd.org Subject: Re: atomic reference counting primatives. Message-ID: <20040520.205403.08940889.imp@bsdimp.com> In-Reply-To: <Pine.BSF.4.21.0405201340590.72391-100000@InterJet.elischer.org> References: <Pine.BSF.4.21.0405201340590.72391-100000@InterJet.elischer.org>
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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?
Warner
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