P1 running...
               x += 4; ==> LOAD  x
                           ADD   #4

 P2:  x++; ==> LOAD x
               ADD  #1
               STORE x

                           STORE x

-------------------------------------------

 P1 running...
               x += 4; ==> LOAD  x
                           ADD   #4

 P2:  x++; ==> WAIT/ BLOCK

                           STORE x

 P2:  x++; ==> LOAD x
               ADD  #1
               STORE x


===========================================


/* shared between P1 and P2 */
int x = 5;
int lock = false;


/* P1 */
while ( 1 )
{
 executeNonCriticalSection();
 while ( lock == true ) {     /* TEST */
   /* wait ... */
 }
****PC***************
 lock = true;                 /* SET */
 executeCriticalSection();
 lock = false;
}


/* P2 */
while ( 1 )
{
 executeNonCriticalSection();
 while ( lock == true ) {
   /* wait ... */
 }
************PC*****************
 lock = true;
 executeCriticalSection();
 lock = false;
}


===========================================


/* shared between P1 and P2 */
int x = 5;
int needLock1 = 0;
int needLock2 = 0;
int turn = 1;  /* P1 */


/* P1 */
while ( 1 )
{
 executeNonCriticalSection();
 needLock1 = 1;
 turn = 2;
 while ( turn == 2 && needLock2 == 1 ) {
   /* wait ... */
 }
 executeCriticalSection();  /* access x */
 needLock1 = 0;
}


/* P2 */
while ( 1 )
{
 executeNonCriticalSection();
 needLock2 = 1;
 turn = 1;
 while ( turn == 1 && needLock1 == 1 ) {
   /* wait ... */
 }
 executeCriticalSection();  /* access x */
 needLock2 = 0;
}