Re: Testing the lws_compare_and_swap_2 syscall

[John David Anglin <dave.anglin@bell.net>]

On 2017-10-25 8:22 PM, Christoph Biedl wrote:
> After staring at lws_compare_and_swap_2 a long time it seems there are
> two issues: First, there is more usage of ",ma" so an update of mem/r26
> hits the wrong place.
The other uses of ",ma" are not a problem as the increment value is 0.  
So, the pointer
is unchanged.  The double word case had an increment of 4 messing up the 
pointer.

Technically, ",ma" with a zero increment provides an ordered load. 
However, the completer
probably isn't necessary as I believe all loads and stores are ordered 
on real hardware.

The "=" completer in the "sub" instructions looks correct to me. When 
*mem and old are
equal, the "b,n     cas2_end" instruction is nullified and new is stored 
in mem.  See comment
from code:

         /*
                 prev = *addr;
                 if ( prev == old )
                   *addr = new;
                 return prev;
         */

In your first test example, old and new should be exchanged in mem. Pass 
should be
mem == new.  Second case should be mem == old.

Dave

-- 
John David Anglin  dave.anglin@bell.net