atomic_dec_and_test() in FreeBSD?

[John Baldwin]

On 26-Mar-2003 Craig Rodrigues wrote:
> On Mon, Mar 24, 2003 at 11:48:10AM -0500, John Baldwin wrote:
>> > http://kernelnewbies.org/documents/kdoc/kernel-api/r287.html
>> 
>> If you strictly need decrement and test, you can use a while loop
>> with atomic_cmpset(), e.g.
>> 
>>         do {
>>                 x = foo;
>>         } while (atomic_cmpset_int(&foo, x, x - 1) == 0);
>>         if (x == 1)
>>                 /* foo just dropped to zero */
>> 
>> What task are you trying to accomplish though?
> 
> 
> I am the port maintainer of the Apache Portable Runtime (apr) library.
> apr has some atomic functions.  For FreeBSD, this is what is defined:
> 
> /**
>  * decrement the atomic variable by 1
>  * @param mem pointer to the atomic value
>  * @return zero if the value is zero, otherwise non-zero
>  */
> int apr_atomic_dec(volatile apr_atomic_t *mem);
> 
> [snip]
> 
>#define apr_atomic_dec(mem)          atomic_subtract_int(mem,1)
> 
> 
> This is obviously quite wrong.
> 
> So are you saying that I should replace this with:
> 
> int apr_atomic_dec(volatile apr_atomic_t *mem);
> 
> int  apr_atomic_dec(volatile apr_atomic_t *mem){
>          apr_atomic_t x
>          do {
>                  x = *mem;
>          } while (atomic_cmpset_int(mem, x, x - 1) == 0);
>          if (x == 1)
>                  /* foo just dropped to zero */
> 
> 
>          ???????
> } 
> 
> Can you give more guidance?

You could do this:

        apr_atomic_t x;

        do {
                x = *mem;
        } while (atomic_cmpset_int(mem, x, x - 1) == 0);
        return (x - 1)

-- 

John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/