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    SubjectRe: zombie with CLONE_THREAD
    The reason strace hangs in that case is an strace bug.  strace is
    blocking in a wait4 call on the specific PID of the zombie group leader,
    which will never report until strace reaps the other non-leader thread
    it is tracing. But strace won't ever do that, because it's blocked
    saying it only cares about the zombie leader's PID. You should report
    that bug in strace. I pity the fool who agreed to maintain that unholy
    pile of spaghetti--what a maroon!

    The kernel bug is that when you kill strace, the non-leader zombie hangs
    around and that keeps the zombie leader around too (which is the one
    you'll see in ps). The following patch fixes that for me. I am not
    100% confident that the locking dance required here doesn't create some
    weird issue (and it certainly seems inefficient how many times the lock
    is released and retaken in this sequence), but maybe 92% sure.


    --- linux-2.6.7-mm4/kernel/exit.c.~1~ 2004-06-30 16:29:06.000000000 -0700
    +++ linux-2.6.7-mm4/kernel/exit.c 2004-06-30 18:55:36.000000000 -0700
    @@ -618,9 +618,21 @@ static inline void forget_original_paren
    reparent_thread(p, father, 0);
    } else {
    ptrace_unlink (p);
    - if (p->state == TASK_ZOMBIE && p->exit_signal != -1 &&
    - thread_group_empty(p))
    - do_notify_parent(p, p->exit_signal);
    + if (p->state == TASK_ZOMBIE) {
    + if (p->exit_signal == -1) {
    + /*
    + * This was only a zombie because
    + * we were tracing it. Now it should
    + * disappear as it would have done
    + * if we hadn't been tracing it.
    + */
    + write_unlock_irq(&tasklist_lock);
    + release_task(p);
    + write_lock_irq(&tasklist_lock);
    + }
    + else if (thread_group_empty(p))
    + do_notify_parent(p, p->exit_signal);
    + }
    list_for_each_safe(_p, _n, &father->ptrace_children) {

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