On jtg-linux3, Volanomark crashed after 28 hours.

This looks like a race between jvmstat and VM shutdown code.
In particular, PeriodicTask::deroll() does not use a lock, it is
possible that the watcher thread is processing a StatSampler task while
the StatSampler::disengage() call from the main thread pulls the bug
out from under it.

To reproduce the bug
1. telnte jtg-linux3
2. export JAVA_HOME
3. /net/jtgb4u4c.sfbay/export/sail8/bigapps/tests/runvmark.ksh -server
4. log files are under /bt/VolanoMark*
check if run.vmark.out is up-to-date. If not, volanomark hungs.

THe libjvm.so is attached.

###@###.### 2002-04-29