CAS is broken

[Tomash Brechko]

On Sun, Nov 11, 2007 at 14:37:07 -0800, Dustin Sallings wrote:
> 	Can you take a look at this and see if it makes any more sense?


> diff --git a/items.c b/items.c
> --- a/items.c
> +++ b/items.c
> @@ -37,6 +38,15 @@ void item_init(void) {
>          tails[i] = NULL;
>          sizes[i] = 0;
>      }
> +}
> +
> +/* Get the next CAS id for a new item. */
> +uint64_t get_cas_id() {
> +    static uint64_t cas_id;
> +    if(cas_id >= MAX_CAS_ID) {
> +        cas_id = 0;
> +    }
> +    return ++cas_id;
>  }

Seems like you want to have a global CAS counter.  This would require
you to have a mutex on it.  Wouldn't it be better to have a CAS local
to each item?  Becase all we need is an unique (key,CAS) pair, we
don't have to have each CAS to be unique by itself.  Besides, this
will decrease the chance of wraparound to effectively zero.


> @@ -129,6 +139,7 @@ item *do_item_alloc(char *key, const siz
>      it->it_flags = 0;
>      it->nkey = nkey;
>      it->nbytes = nbytes;
> +    it->cas_id = get_cas_id();

Then this would be it->cas_id = 0;, 'gets' will return current value,
and 'cas' will do ++it->cas_id; (no need to have MAX_CAS_ID, C
guarantees no error on wraparound of unsigned types, not to say that
it will never happen).


-- 
   Tomash Brechko