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

[Christian Brauner]

On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn <jannh@xxxxxxxxxx> wrote:
>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.

The latter case probably being preferred if this API is supposed to be useable for thread management in userspace.