> If thread t1 locks mutex m, can thread t2 unlock it?
> I ask because I get no sign of exceptions being thrown when 
> I execute the following code in the toplevel:
> [...]
> Is this a bug or a feature? Are threads supposed to be able 
> to unlock mutexes held by _other_ threads?

No, but this isn't necessarily checked.

The bytecode-level implementation of the thread library doesn't check,
and will actually work correctly if thread A locks the mutex and
thread B unlocks it.  

The system-level implementation of the thread library is a thin
wrapper around POSIX or Win32 mutexes.  The POSIX spec says that the
behavior of unlocking by another thread than the one that locked is
undefined: some implementations can signal an error, others can "do
the right thing", others can crash.

- Xavier Leroy

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