linux-kernel - Re: [path][rfc] add PR_DETACH prctl command [3/3]

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Message-ID: <20110419162909.GC23914@redhat.com>
Date:	Tue, 19 Apr 2011 18:29:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Stas Sergeev <stsp@...et.ru>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [path][rfc] add PR_DETACH prctl command [3/3]

On 04/19, Oleg Nesterov wrote:
>
> I'll try to check these patches from the correctness pov tomorrow,
> but to be honest I hope someone will nack them before I start ;)

OK, I briefly looked at 3/3. It looks certainly wrong.

	notif = do_signal_parent(...);
	if (notif != DEATH_REAP) {
	....

do_signal_parent() must not return DEATH_REAP (this means that
leader->exit_signal becomes -1), but this can happen and this is bug.

I can be wrong, but iirc this was already discussed, probably when
you sent the very first version which had the same problem. That is
why (in particular) do_notify_parent() does

	BUG_ON(tsk->group_leader != tsk || !thread_group_empty(tsk))

but you simply removed this check to hide the problem.

Also. I didn't actually read the patch yet, but iiuc: if a task T does
PR_DETACH and then exits, init can't reap it until the old parent does
wait. This can confuse the poor admin, he can see the zombies with
ppid = 1 and there is no way to understand why.

Oleg.

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