linux-kernel - Re: [RFC PATCH 2/3] exec: don't wait for zombie threads with cred_guard

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Message-ID: <aRIAEYH2iLLN-Fjg@redhat.com>
Date: Mon, 10 Nov 2025 16:09:05 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Cyrill Gorcunov <gorcunov@...il.com>
Cc: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
	linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] exec: don't wait for zombie threads with
 cred_guard_mutex held

Hi Cyrill,

On 11/10, Cyrill Gorcunov wrote:
>
> On Sun, Nov 09, 2025 at 06:15:33PM +0100, Oleg Nesterov wrote:
> ..
> > static int kill_sub_threads(struct task_struct *tsk)
> > {
> >  	struct signal_struct *sig = tsk->signal;
> > 	int err = -EINTR;
> >
> > 	read_lock(&tasklist_lock);
> > 	spin_lock_irq(&tsk->sighand->siglock);
> > 	if (!((sig->flags & SIGNAL_GROUP_EXIT) || sig->group_exec_task)) {
> > 		sig->group_exec_task = tsk;
> > 		sig->notify_count = -zap_other_threads(tsk);
>
> Hi Oleg! I somehow manage to miss a moment -- why negative result here?

You know, initially I wrote

		sig->notify_count = 0 - zap_other_threads(tsk);

to make it clear that this is not a typo ;)


This is for exit_notify() which does

	/* mt-exec, de_thread() -> wait_for_notify_count() */
	if (tsk->signal->notify_count < 0 && !++tsk->signal->notify_count)
		wake_up_process(tsk->signal->group_exec_task);

Then setup_new_exec() sets notify_count > 0 for __exit_signal() which does

	/* mt-exec, setup_new_exec() -> wait_for_notify_count() */
	if (sig->notify_count > 0 && !--sig->notify_count)
		wake_up_process(sig->group_exec_task);

Yes this needs more comments and (with or without this patch) cleanups.
Note that exit_notify() and __exit_signal() already (before this patch)
use ->notify_count almost the same way, just exit_notify() assumes that
notify_count < 0 means the !thread_group_leader() case in de_thread().

Oleg.