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Independent Reads of Independent Writes - IRIW

• PPC is not "multiple copy atomic".
• A store is not propagated to all other threads at the same time.
• This breaks the IRIW test of the torture test suite.

  thread 1            thread 2         thread 3          thread 4
   a = x               x = 1            y = 1             b = y       
   b = y                                                  a = x    

a==1 && b==0                                            a==0 && b==1      

IRIW - standard HostSpot barriers

• PPC is not "multiple copy atomic".
• A store is not propagated to all other threads at the same time.
• This breaks the IRIW test of the torture test suite.

  thread 1            thread 2         thread 3          thread 4
   a = x               x = 1            y = 1             b = y       
   b = y              StoreLoad       StoreLoad           a = x    

a==1 && b==0                                            a==0 && b==1      

IRIW - PPC solution

• PPC is not "multiple copy atomic".
• A store is not propagated to all other threads at the same time.
• This breaks the IRIW test of the torture test suite.

  thread 1            thread 2         thread 3          thread 4  
 StoreLoad                                              StoreLoad
   a = x               x = 1            y = 1             b = y       
 StoreLoad                                              StoreLoad
   b = y                                                  a = x    

• 8029101: PPC64: ordering of Independent Reads of Independent Writes
• Similar issue in taskqueue:
• 8012144: multiple SIGSEGVs fails on staxf

Initialize volatile fields in constructors

public class AtomicInteger {
    private volatile int value;

    public AtomicInteger(int initialValue) {
        value = initialValue;
    }
}

Thread 1:

AtomicInteger a = new AtomicInteger(42);

Thread 2:

if (a != null) return addAndGet(1); // May return 1

Returning "1" is legal Java code.

It will never happen on x86/sparc.

The ppc64 port avoids this, too, by adding an additional barrier.