If one thread uses ptrace on another thread in the same thread group, there
can be a deadlock when calling exec.  The ptrace_stop change ensures that
no tracing stop can be entered for a queued signal, or exit tracing, if the
tracer is part of the same dying group.  The exit_notify change prevents a
ptrace zombie from sticking around if its tracer is in the midst of a group
exit (which an exec fakes), so these zombies don't hold up de_thread's
synchronization.  The de_thread change ensures the new thread group leader
doesn't wind up ptracing itself, which would produce its own deadlocks.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>