Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

[Alexei Starovoitov]

On Tue, Jun 30, 2020 at 07:29:34AM -0500, Eric W. Biederman wrote:
> 
> diff --git a/net/bpfilter/bpfilter_kern.c b/net/bpfilter/bpfilter_kern.c
> index 91474884ddb7..3e1874030daa 100644
> --- a/net/bpfilter/bpfilter_kern.c
> +++ b/net/bpfilter/bpfilter_kern.c
> @@ -19,8 +19,8 @@ static void shutdown_umh(void)
>         struct pid *tgid = info->tgid;
>  
>         if (tgid) {
> -               kill_pid_info(SIGKILL, SEND_SIG_PRIV, tgid);
> -               wait_event(tgid->wait_pidfd, !pid_task(tgid, PIDTYPE_TGID));
> +               kill_pid(tgid, SIGKILL, 1);
> +               wait_event(tgid->wait_pidfd, !pid_has_task(tgid, PIDTYPE_TGID));
>                 bpfilter_umh_cleanup(info);
>         }
>  }
> 
> > And then did:
> > while true; do iptables -L;rmmod bpfilter; done
> >  
> > Unfortunately sometimes 'rmmod bpfilter' hangs in wait_event().
> 
> Hmm.  The wake up happens just of tgid->wait_pidfd happens just before
> release_task is called so there is a race.  As it is possible to wake
> up and then go back to sleep before pid_has_task becomes false.
> 
> So I think I need a friendly helper that does:
> 
> bool task_has_exited(struct pid *tgid)
> {
> 	bool exited = false;
> 
> 	rcu_read_lock();
>         tsk = pid_task(tgid, PIDTYPE_TGID);
>         exited = !!tsk;
>         if (tsk) {
>         	exited = !!tsk->exit_state;
> out:
> 	rcu_unlock();
> 	return exited;
> }

All makes sense to me.
If I understood the race condition such helper should indeed solve it.
Are you going to add such patch to your series?
I'll proceed with my work on top of your series and will ignore this
race for now, but I think it should be fixed before we land this set
into multiple trees.