Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

On Sat, Mar 14, 2015 at 08:24:56PM +0100, Oleg Nesterov wrote:

On 03/14, Josh Triplett wrote:

quoted

On Sat, Mar 14, 2015 at 03:35:58PM +0100, Oleg Nesterov wrote:

quoted

On 03/12, Josh Triplett wrote:

quoted

@@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	if (group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);

-	if (unlikely(tsk->ptrace)) {
+	if (tsk->autoreap) {
+		autoreap = true;
+	} else if (unlikely(tsk->ptrace)) {
 		int sig = thread_group_leader(tsk) &&
 				thread_group_empty(tsk) &&
 				!ptrace_reparented(tsk) ?

@@ -612,8 +616,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	}

 	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
-	if (tsk->exit_state == EXIT_DEAD)
+	if (tsk->exit_state == EXIT_DEAD) {
 		list_add(&tsk->ptrace_entry, &dead);
+		clonefd_do_notify(tsk);
+	}

And even ignoring semantics issues, this change looks simply buggy anyway ;)

How can we do list_add(&tsk->ptrace_entry) if it is traced by _another_ task?
->ptrace_entry is used by debugger.

That list_add was there before; I didn't change that.

But this doesn't matter,

quoted

I just added a
second line inside the EXIT_DEAD case, to call clonefd_do_notify (which
wakes up potential callers of poll/read).

No. Please read this code before and after your patch. You also added

	if (tsk->autoreap)
		autoreap = true;

at the start. At this can trigger the _wrong_ list_add(&tsk->ptrace_entry),
when the task is traced by another thread.

The current code can only use ->ptrace_entry if it was untraced (by us).

Ugh.  I finally realized just how magic the logic is there; thanks for
catching this.  The call to forget_original_parent at the top of
exit_notify can potentially add the process to the list "dead" here,
either in exit_ptrace() or in reparent_leader() (the latter of which has
its own duplicate of part of exit_notify's logic, including
do_notify_parent and setting exit_state).  Then exit_notify can add the
task to "dead" itself under some conditions that clearly depend on the
exact nature of the existing three-way conditional above.  And finally
exit_notify loops over "dead" and releases all the tasks there.

I'll investigate this further and make sure the ptrace case gets
handled correctly.

- Josh Triplett