atomic_dec_and_test() in FreeBSD?

[Garrett Rooney]

John Baldwin wrote:

>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);
>
>  
>

I believe Craig is looking for a user space solution, not kernel space.

-garrett