The correct operation of this queue relies on the fact that at most one thread in the system will wait for a particular process to exit. The reason for this restriction is that Unix semantics only allow us to perform a waitpid() once for a given process id. VMProcess uses Java synchronization to enforce this property.

Note that a strange issue arises on Linux: it is only possible to wait for a child process to exit from the pthread that forked the process. (This due to the threading model used by Linux, where pthreads have some characteristics of processes.) Because of this limitation, in order for a Java thread to wait for a process to exit, it must migrate to the Processor that created the process. A ProcessorLock in the virtual processor object protects access to this queue, since it may be accessed from another Processor. When polling for exited processes, we have to be careful to ignore GreenThreads that are still being dispatched on another virtual processor.

I would imagine that AIX pthreads work correctly with respect to allowing arbitrary pthreads to perform a waitpid().