On Mar 13, 2007, at 5:42 PM, Bang Jun-Young wrote:

> On 3/13/07, Jason Thorpe <thorpej@shagadelic.org> wrote:
>> The API is borrowed from Solaris.  There is one major difference,
>> however.  Solaris defines "cas" as COMPARE-AND-SWAP, returning the
>> previous value of the memory cell.  I have defined it as COMPARE-AND-
>> STORE, returning true if the ops succeeds, or false otherwise.  The
>> reason for this is because some platforms will have extreme  
>> difficulty
>> implementing COMPARE-AND-SWAP, whereas COMPARE-AND-STORE is somewhat
>> easier for these platforms.
>
> uint32_t
> atomic_compare_and_swap_32(volatile uint32_t *target, uint32_t cmp,
> uint32_t new)
> {
>    uint32_t old = *target;
>    (void) atomic_compare_and_store_32(target, cmp, new);
>    return old;
> }
>
> I think atomic_compare_and_swap_*() might not be so difficult to  
> implement or
> expensive as you expected. :-)

You're missing the memory barrier before fetching the old target.   
Extra memory barriers are expensive.

I actually discussed this with a former Sun person while carpooling  
home tonight.  Seems like this is mostly intended only for programmer  
convenience as opposed to any real need.

-- thorpej