JVM: 1.5.0_07
kernel: 2.6.17-1.2142_FC4smp
glibc: 2.3.6-3

Found filed , but closed, bug report at Sun:
       http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6359295

Strack trace:

sun.jvm.hotspot.debugger.DebuggerException: sun.jvm.hotspot.debugger.DebuggerException: get_thread_regs failed for a lwp
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.execute(LinuxDebuggerLocal.java:134)
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.getThreadIntegerRegisterSet(LinuxDebuggerLocal.java:437)
       at sun.jvm.hotspot.debugger.linux.LinuxThread.getContext(LinuxThread.java:48)
       at sun.jvm.hotspot.runtime.linux_x86.LinuxX86JavaThreadPDAccess.getCurrentFrameGuess(LinuxX86JavaThreadPDAccess.java:75)
       at sun.jvm.hotspot.runtime.JavaThread.getCurrentFrameGuess(JavaThread.java:252)
       at sun.jvm.hotspot.runtime.JavaThread.getLastJavaVFrameDbg(JavaThread.java:211)
       at sun.jvm.hotspot.tools.PStack.initJFrameCache(PStack.java:167)
       at sun.jvm.hotspot.tools.PStack.run(PStack.java:37)
       at sun.jvm.hotspot.tools.JStack.run(JStack.java:41)
       at sun.jvm.hotspot.tools.Tool.start(Tool.java:204)
       at sun.jvm.hotspot.tools.JStack.main(JStack.java:58)
Caused by: sun.jvm.hotspot.debugger.DebuggerException: get_thread_regs failed for a lwp
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.getThreadIntegerRegisterSet0(Native Method)
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.access$800(LinuxDebuggerLocal.java:34)
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$1GetThreadIntegerRegisterSetTask.doit(LinuxDebuggerLocal.java:431)
       at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.run(LinuxDebuggerLocal.java:109)

What is strange is that this has not worked before. So last night, I upgrading my kernel and glibc,
rebooted, tried jstack again, and it worked. Now came back this morning, tried it again,
and it is not working anymore again.