atomic_dec_and_test() in FreeBSD?

[John Baldwin]

On 04-Apr-2003 Craig Rodrigues wrote:
> On Wed, Mar 26, 2003 at 12:13:21PM -0500, John Baldwin wrote:
>> > 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)
> 
> 
> 
> This macro exists on -CURRENT, but I can't seem to find it
> on FreeBSD 4.7.  What is the equivalent solution on that platform?

4.7 doesn't have a real good equivalent.  However, on 4.7 you can
probably just do:

        s = splhigh();
        x--;
        zero = x == 0;
        splx(s);
        if (x == 0)
                /* refcount is zero */

In fact, depending on what you do in the zero case, you probably
would need to do:

        s = splhigh();
        if (--x == 0)
                /* refcount is zero, call free(), etc. */
        splx(s);

-- 

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