Re: [PATCH] pid: use READ_ONCE() in pid_alive()

[Oleg Nesterov <oleg@redhat.com>]

On 10/04, David Laight wrote:
>
> On Sun, 4 Oct 2026 12:55:34 +0200
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Yep. That is why do_each_pid_task() needs tasklist_lock.
> >
> > This is the known fact, let me quote the part of my old email
> > https://lore.kernel.org/all/20200512150936.GA28621@redhat.com/
> >
> > 	> Currently the tasklist_lock is shared mainly in order to observe
> > 	> the list atomically for the PRIO_PGRP and PRIO_USER cases, as
> > 	> the actual lookups are already rcu-safe,
> >
> > 	not really...
> >
> > 	do_each_pid_task(PIDTYPE_PGID) can race with change_pid(PIDTYPE_PGID)
> > 	which moves the task from one hlist to another. Yes, it is safe in
> > 	that task_struct can't go away. But still this is not right because
> > 	do_each_pid_task() can scan the wrong (2nd) hlist.
> >
> > Somehow I thought this was documented, but it isn't. And this is not obvious.
> > I think this deserves a comment above do_each_pid_task(), will send the patch.
>
> I guess the rcu protection lets the task exit without holding the lock?
> Is that really significant given the other things that happen during task exit.

Sorry, I don't understand your question...

> Could do_each_pid_task() use hlist_nulls_for_each_entry_rcu() and rescan
> if it got the wrong terminator.
> Or does scanning twice cause grief as well.

I don't think it can. Say, __kill_pgrp_info() is a "typical" user of
do_each_pid_task(). What can it do if it detects that get_nulls_value()
doesn't match after the main loop? The signal was already sent.

Oleg.