reference counting.. continued..

[Julian Elischer]

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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