Klaus Dittrich observed this bug and posted a test case for it.

This patch fixes both that failure mode and some others possible.  What
Klaus saw was a false negative (i.e.  ECHILD when there was a child)
when the group leader was a zombie but delayed because other children
live; in the test program this happens in a race between the two threads
dying on a signal.

The change to the TASK_TRACED case avoids a potential false positive
(blocking, or WNOHANG returning 0, when there are really no children
left), in the race condition where my_ptrace_child returns zero.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>