linux-kernel - Re: [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Mon, 5 Jun 2023 15:48:44 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mike Christie <michael.christie@...cle.com>
Cc:     linux@...mhuis.info, nicolas.dichtel@...nd.com, axboe@...nel.dk,
        ebiederm@...ssion.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [PATCH 1/1] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

On 06/01, Mike Christie wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p)
>  
>  	while_each_thread(p, t) {
>  		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
> -		count++;
> +		/* Don't require de_thread to wait for the vhost_worker */
> +		if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)
> +			count++;

Well if you do this, then you should also change __exit_signal() to
not decrement sig->notify_count. Otherwise de_thread() can succeed
before the "normal" sub-threads exit.

But this can't be right anyway... If nothing else, suppose we have
a process with 3 threads:

	M	- the main thread, group leader
	T	- sub-thread
	V	- vhost worker

T does exec and calls de_thread().

M exits. T takes the leadership and does release_task()

V is still running but V->group_leader points to already freed M.

Or unshare_sighand() after that... If nothing else this means that
lock_task_sighand(T) and lock_task_sighand(V) will take different
locks.

Oleg.