On 04/18, Jann Horn wrote:
On Wed, Apr 17, 2019 at 3:09 PM Oleg Nesterov oleg@redhat.com 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, sorry, I meant /proc/pid/task/sub-thread-tid.
But poll("/proc/sub-thread-tid") won't work too, we can't rely on do_notify_parent() if the task is not a 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.
Heh, I didn't even notice that WARN_ON_ONCE(task && !thread_group_leader(task)) ;)
@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 last version of CLONE_PIDFD doesn't allow CLONE_THREAD, so we can forget about this problem for now.
Oleg.