[cpp-threads] Implicit Synchronization

[Doug Lea]

Lawrence Crowl wrote:
> One of the things we must make sure we address is the implicit
> syncronization that we expect in the data that we guard with
> locks.  For example, with double-checked locking for sharing
> variable initialization, we cannot define only the
> synchronization on the locks, but also on the variable being
> initialized.  Likewise, for reference-counted objects, we must
> know that on (atomically) decrementing the count to zero,
> reading the pointer must result in the value written.
> 

This is a very good point, that we encounter all the time
developing java.util.concurrent: You often need heavier
barriers than you'd otherwise expect, to make sure that
if some less-than-fully thread-safe object is, for example,
placed in a hash table by one thread, that it is guaranteed
readable by others retrieving it.

You can say all you want in specs that people shouldn't expect
this to work, but they will, and so unless you do this part, users
will encounter what they claim are bugs.

-Doug