Re: [PATCH 0/3] bpf: kernel/bpf/task_iter.c: don't abuse next_thread()

[Oleg Nesterov <oleg@redhat.com>]

On 11/15, Yonghong Song wrote:
>
> On 11/14/23 11:32 AM, Oleg Nesterov wrote:
> >Compile tested.
> >
> >Every lockless usage of next_thread() was wrong, bpf/task_iter.c is
> >the last user and is no exception.
>
> It would be great if you can give more information in the commit message
> about why the usage of next_thread() is wrong in bpf/task_iter.c.

I tried to explain the problems in the changelogs:

1/3:
	task_group_seq_get_next() can return the group leader twice if it races
	with mt-thread exec which changes the group->leader's pid.

2/3:
	bpf_iter_task_next() can loop forever, "kit->pos == kit->task" can never
	happen if kit->pos execs.

> IIUC, some information is presented in :
>   https://lore.kernel.org/all/20230824143112.GA31208@redhat.com/

Yes, Linus and Eric suggest to simply kill next_thread(). I am not
sure, this needs another discussion.

But as for bpf/task_iter.c... Even _if_ the usage was correct, this
code simply doesn't need the "circular" next_thread(), NULL at the
end simplifies the code.

> Also, please add 'bpf' in the subject tag ([PATCH bpf 0/3]) to
> make it clear the patch should be applied to bpf tree.

OK, will do next time. Or should I resend this series with 'bpf'
in the subject tag?

Thanks,

Oleg.