Re: [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped.

Oleg Nesterov [off-list ref] writes:

On 03/03, Eric W. Biederman wrote:

quoted

@@ -1065,11 +1065,8 @@ static int de_thread(struct task_struct *tsk)
 	}

 	sig->group_exit_task = tsk;
-	sig->notify_count = zap_other_threads(tsk);
-	if (!thread_group_leader(tsk))
-		sig->notify_count--;
-
-	while (sig->notify_count) {
+	zap_other_threads(tsk);
+	while (atomic_read(&sig->live) > 1) {
 		__set_current_state(TASK_KILLABLE);
 		spin_unlock_irq(lock);
 		schedule();

Very nice. So de_thread() returns as soon as all other threads decrement
signal->live in do_exit(). Before they do, say, exit_mm(). This is already
wrong, for example this breaks OOM. Plus a lot more problems afaics,  but
lets ignore this.

Which means that we need to keep sig->notify_count.

Note that de_thread() also unshares ->sighand before return. So in the
case of mt exec it will likely see oldsighand->count != 1 and alloc the
new sighand_struct and this breaks the locking.

Because the execing thread will use newsighand->siglock to protect its
signal_struct while the zombie threads will use oldsighand->siglock to
protect the same signal struct. Yes, tasklist_lock + the fact irq_disable
implies rcu_lock mostly save us but not entirely, say, a foreign process
doing __send_signal() can take the right or the wrong lock depending on
/dev/random.

Which leads to the question how can we get back tot he 2.4 behavior
of freeing sighand_struct in do_exit?

At which point as soon as we free sighand_struct if we are the last
to dying thread notify de_thread and everything works.

There are only two uses of sighand->siglock that I can see in
release_task:  __ptrace_unlink and __exit_signal.

For what __ptrace_unlink is doing we should just be able to skip
acquiring of siglock if PF_EXITING is set.

__exit_signal is a little more interesting but half of what it is
doing looks like it was pulled out of do_exit and just needs to
be put back.

Which probably adds up to 4 or 5 small carefully written patches to sort
out that part of the exit path, but I think it leads to a very simple
and straight forward result.  With the only side effect that we
can exec a process and still have a debugger reaping zombie threads.

Oleg does that sound reasonable to you?

Eric