On April 18, 2019 7:23:38 PM GMT+02:00, Jann Horn jannh@google.com 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, 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.