Re: [PATCH RFC 1/2] Add polling support to pidfd

[Jann Horn]

On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 04/16, Joel Fernandes wrote:
> > On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote:
> > >
> > > Could you explain when it should return POLLIN? When the whole process exits?
> >
> > It returns POLLIN when the task is dead or doesn't exist anymore, or when it
> > is in a zombie state and there's no other thread in the thread group.
>
> IOW, when the whole thread group exits, so it can't be used to monitor sub-threads.
>
> just in case... speaking of this patch it doesn't modify proc_tid_base_operations,
> so you can't poll("/proc/sub-thread-tid") anyway, but iiuc you are going to use
> the anonymous file returned by CLONE_PIDFD ?

I don't think procfs works that way. /proc/sub-thread-tid has
proc_tgid_base_operations despite not being a thread group leader.
(Yes, that's kinda weird.) AFAICS the WARN_ON_ONCE() in this code can
be hit trivially, and then the code will misbehave.

@Joel: I think you'll have to either rewrite this to explicitly bail
out if you're dealing with a thread group leader, or make the code
work for threads, too.