Re: [PATCH]: linux and zombie threads

[Mark Kettenis <kettenis at science dot uva dot nl>]

Michael Snyder <msnyder@cygnus.com> writes:

> Mark -- these changes prepare the way for a libthread_db change
> that will allow gdb to recognize zombie threads.

Hmm, what do you mean with zombie threads when you're talking about a
libthread_db change?

For clarity, it seems to be two concepts of "zombie" related to
LinuxThreads.  One is which I would call a "zombie thread", which
basically is a thread that has exited (by invoking pthread_exit() or
returning from the, but has not yet been joined.  These are reported
by libthread_db as TD_THR_ZOMBIE.

The other I would call a "zombie process", which basically is a kernel
thread that has exited (by invoking _exit()) but has not yet been
waited for.  These are the ondes reported as TD_THR_UNKNOWN.  Their
existence is very Linux-specific, and the fact that they tend to show
up when debugging is related to a kernel bug for which I have been
unable to find a kernel hacker to fix.

> 2001-05-22  Michael Snyder  <msnyder@redhat.com>
> 
> 	* thread-db.c: Allow for defunct zombie threads.	
> 	(attach_thread): Do not attempt to attach zombie thread.
> 	(thread_db_thread_alive): Return false for defunct zombie thread.
> 	(find_new_threads_callback): Don't add defunct zombie thread to list.

That said, I think your patch is OK.  We want GDB to ignore the
"zombie processes".  Practically all Linux kernels contain the bug
that creates them, and even if the bug was fixed, there probably is a
small window where these "zombie processes" are visible.

Could you clean up your patch a bit before checking it in?  There seem
to be some whitespace/indentation problems.  And lining up the local
variables in thread_db_thread_alive() seems to be against the coding
standards too (since it introduces unnecessary whitespace).

Mark