Subj : experimental i686 Peterson's algorithm impl...
To   : comp.programming.threads
From : Chris Thomasson
Date : Thu Apr 07 2005 01:19 pm

This mutex is faster than a traditional spinlock because it does not have to 
use the LOCK prefix and it doesn't use atomic operations ( cmpxchg, xchg, 
ect... ). However, it does have a single (store/load) for the locking 
functions. This is necessary to prevent x86 from reordering 
load-after-store. Also, it is only good for two processes or threads.




This is experimental for now, but the code should work just fine:




typedef signed int ac_i686_intword_t;


/* must be three adjacent words and aligned on 128 boundary */
typedef struct
__attribute__( (packed) )
__attribute__( (aligned( 128 )) )
ac_i686_mutex_
{
  ac_i686_intword_t m1;
  ac_i686_intword_t m2;
  ac_i686_intword_t t;

} ac_i686_mutex_t;




#define ac_i686_mutex_init( ac_macro_this ) \
  (ac_macro_this)->m1 = 0; \
  (ac_macro_this)->m2 = 0; \
  (ac_macro_this)->t = 1


extern void ac_i686_mutex_lock_1( ac_i686_mutex_t* );
extern void ac_i686_mutex_unlock_1( ac_i686_mutex_t* );
extern void ac_i686_mutex_lock_2( ac_i686_mutex_t* );
extern void ac_i686_mutex_unlock_2( ac_i686_mutex_t* );




align 16
ac_i686_mutex_lock_1 PROC
  mov eax, [esp + 4]
  mov ecx, 1
  mov [eax], ecx
  mov ecx, 2
  mov [eax + 8], ecx
  mfence ; load-after-store

ac_i686_mutex_lock_1_retry:
  mov ecx, 0
  cmp ecx, [eax + 4]
  je ac_i686_mutex_lock_1_done
  pause
  mov ecx, 1
  cmp ecx, [eax + 8]
  je ac_i686_mutex_lock_1_done
  pause
  jmp ac_i686_mutex_lock_1_retry

ac_i686_mutex_lock_1_done:
  ret
ac_i686_mutex_lock_1 ENDP




align 16
ac_i686_mutex_unlock_1 PROC
  mov ecx, [esp + 4]
  mov eax, 0
  mov [ecx], eax
  ret
ac_i686_mutex_unlock_1 ENDP




align 16
ac_i686_mutex_lock_2 PROC
  mov eax, [esp + 4]
  mov ecx, 1
  mov [eax + 4], ecx
  mov ecx, 1
  mov [eax + 8], ecx
  mfence ; load-after-store

ac_i686_mutex_lock_2_retry:
  mov ecx, 0
  cmp ecx, [eax]
  je ac_i686_mutex_lock_2_done
  pause
  mov ecx, 2
  cmp ecx, [eax + 8]
  je ac_i686_mutex_lock_2_done
  pause
  jmp ac_i686_mutex_lock_2_retry

ac_i686_mutex_lock_2_done:
  ret
ac_i686_mutex_lock_2 ENDP




align 16
ac_i686_mutex_unlock_2 PROC
  mov eax, [esp + 4]
  mov ecx, 0
  mov [eax + 4], ecx
  ret
ac_i686_mutex_unlock_2 ENDP


-- 
http://appcore.home.comcast.net/
(portable lock-free data-structures)

.