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 ?
Then all you need is
!task || task->exit_state && thread_group_empty(task)
Yes this works as well, all the tests pass with your suggestion so I'll change it to that. Although I will the be giving up returing EPOLLERR if the task_struct doesn't exit. We don't need that, but I thought it was cool to return it anyway.
OK, task == NULL means that it was already reaped by parent, pid_nr is free, probably useful....
Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably live in task->signal_struct.
About wait_pidfd living in signal_struct, that wont work since the waitqueue has to survive for the duration of the poll system call.
That is why I said this will need the additional cleanup in free_signal_struct(). But I was wrong, somehow I forgot that free_poll_entry() needs wq_head->lock ;) so this will need much more complications, lets forget it...
Also the waitqueue living in struct pid solves the de_thread() issue I mentioned later in the following thread and in the commit message: https://lore.kernel.org/patchwork/comment/1257175/
Hmm...
2. By including the struct pid for the waitqueue means that during de_exec, the thread doing de_thread() automatically gets the new waitqueue/pid even though its task_struct is different.
this one?
this is not true, or I do not understand...
it gets the _same_ (old, not new) PIDTYPE_TGID pid even if it changes task_struct. But probably this is what you actually meant, because this is what your patch wants or I am totally confused.
And note that exec/de_thread doesn't change ->signal_struct, so I do not understand you anyway. Nevermind.
Oleg.