Question

The java meomry model mandates that synchronize blocks that synchronize on the same monitor enforce a before-after-realtion on the variables modified within those blocks. Example:

// in thread A
synchronized( lock )
{
  x = true;
}

// in thread B
synchronized( lock )
{
  System.out.println( x );
}

In this case it is garanteed that thread B will see x==true as long as thread A already passed that synchronized-block. Now I am in the process to rewrite lots of code to use the more flexible (and said to be faster) locks in java.util.concurrent, especially the ReentrantReadWriteLock. So the example looks like this:

EDIT: The example was broken, because I incorrectly transformed the code, as noted by matt b. Fixed as follows:

// in thread A
lock.writeLock().lock();
{
  x = true;
}
lock.writeLock().unlock();

// in thread B
lock.readLock().lock();
{
  System.out.println( x );
}
lock.readLock().unlock();

However, I have not seen any hints within the memory model specification that such locks also imply the nessessary ordering. Looking into the implementation it seems to rely on the access to volatile variables inside AbstractQueuedSynchronizer (for the sun implementation at least). However this is not part of any specification and moreover access to non-volatile variables is not really condsidered covered by the memory barrier given by these variables, is it?

So, here are my questions:

• Is it safe to assume the same ordering as with the “old” synchronized blocks?
• Is this documented somewhere?
• Is accessing any volatile variable a memory barrier for any other variable?

Regards,
Steffen

—

Comment to Yanamon:

Look at the following code:

// in thread a
x = 1;
synchronized ( a ) { y = 2; }
z = 3;

// in thread b
System.out.println( x );
synchronized ( a ) { System.out.println( y ); }
System.out.println( z );

From what I understood, the memory barrier enforces the second output to show 2, but has no guaranteed affect on the other variables…? So how can this be compared to accessing a volatile variable?

Answer 1

From the API-doc:

All Lock implementations must enforce
the same memory synchronization
semantics as provided by the built-in
monitor lock, as described in The
Java Language Specification, Third
Edition (17.4 Memory Model):

* A successful lock operation has the same memory synchronization effects as a successful Lock action.
* A successful unlock operation has the same memory synchronization effects as a successful Unlock action.

Unsuccessful locking and unlocking
operations, and reentrant
locking/unlocking operations, do not
require any memory synchronization
effects.