Date: Wed, 9 Jun 2004 11:46:09 -0700 (PDT) From: Julian Elischer <julian@elischer.org> To: arch@freebsd.org Subject: Re: reference counting.. continued.. Message-ID: <Pine.BSF.4.21.0406091144310.57882-100000@InterJet.elischer.org> In-Reply-To: <Pine.BSF.4.21.0406091124120.57882-100000@InterJet.elischer.org>
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Eak! was I on something
my head said "ref-count" and my fongers typed "schedlock"
s/schedlock/ref-count/
On Wed, 9 Jun 2004, Julian Elischer wrote:
>
> There are several goals that need to be kept in mind.
>
> * Operations should be quick..
> * Atomic operations can be slow in some architectures.
> * the last free may need to occur at an inconvenient time.
> (in sched-lock or an interrupt routine).
> * There may need to be more than one kind of schedlock,
> for different cases, though I'd hope that they'd have the same
> methods..
> (A reference count scheme that doesn't have to ever consider the
> fact that the object may be freed in an interrupt handler
> might be considereably smaller/quicker than one that needs to
> take that possibility into account. )
>
> Here's an API I mentionned once before.. I'm only showing this as an
> example to start discussion...
> it doesn't cover everything we'd need..
>
> For example how would you do deferred free?
>
>
> ------------------quoted old email------------------
>
> I have seen and liked
> a refcount API which was (from memory something like):
>
> void * refcnt_add(offsetof(struct obj, refcnt), void ** object_p)
>
> which takes a pointer to the object pointer you are copying, and
> atomically increments it and returns the contents of the pointer.
> If the contents of the pointer are NULL, then it returns NULL
> and doesn't increment anything..
>
> The reference decrement atomically reduced the reference count and
> zapped the pointer, and retunred a copy of the pointer if
> the reference count had gone to 0 (or NULL if not).
>
> So usage was:
> struct xx *globalpointer; /* has its own owner somewhere */
>
> mypointer = refcnt_add(offsetof(xx, refcnt), globalptr)
> if (mypointer == NULL) {
> printf("didn't find an object\n"
> return (-1);
>
> }
> manipulate(mypointer)
> if ((tmppointer = refcnt_drop(&mypointer, &globalpointer))) {
> free(tmppointer);
> }
>
>
>
> someone else who owns the globalpointer reference might might in
> the meanwhile do:
>
> if ((tmppointer = refcnt_drop(globalpointer->refcnt, &globalpointer))) {
> free(tmppointer);
> }
>
> and you were guaranteed to get a predictable result.
>
>
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