Valid reorderings - under new JMM

问题

I just wanted to know whether the below reorderings are valid one or not under new JMM model

Original Code: 
     instanceVar1 = value ;//  normal read operation, no volatile
     synchronized(this) {
       instanceVar2 = value2; //normal read operation, no volatile   
     }
     instanceVar3 = value3;  //normal read operation, no volatile

The above code can be reordered into the following executions.

Case 1:

     synchronized(this) {
       instanceVar2 = value2; //normal read operation, no volatile   
       instanceVar1 = value ;//  normal read operation, no volatile
     }
     instanceVar3 = value3;  //normal read operation, no volatile

Another case :

Case 2:

  synchronized(this) {
       instanceVar3 = value3;  //normal read operation, no volatile
       instanceVar2 = value2; //normal read operation, no volatile   
       instanceVar1 = value ;//  normal read operation, no volatile
     }

Another Case :

Case 3: 

    instanceVar3 = value3;  //normal read operation, no volatile
    synchronized(this) {
       instanceVar2 = value2; //normal read operation, no volatile   
       instanceVar1 = value ;//  normal read operation, no volatile
     }

Another Case :

Case 4: 

    instanceVar3 = value3;  //normal read operation, no volatile
    synchronized(this) {
       instanceVar2 = value2; //normal read operation, no volatile   
     }
    instanceVar1 = value ;//  normal read operation, no volatile

Do all the above 4 cases are valid reordering of original Code under new JMM Model. I have given all the above reorderings based on my understanding of http://gee.cs.oswego.edu/dl/jmm/cookbook.html

回答1:

Consider how the normal load/stores are reordered with the monitor enter and exits:

Case 1 reorders a normal load/store with a monitor enter which is a valid reordering.

Case 2 reorders a normal load/store with a monitor enter, and a monitor exit followed by a normal load/store, which are valid reorderings.

See a similar example: Roach Motels and Java Memory Model. This shows that accesses can be moved into a synchronized block but not back out again.

Case 3 and 4 reorders a monitor enter followed by a normal load/store which is not valid.

来源：https://stackoverflow.com/questions/17505830/valid-reorderings-under-new-jmm

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