Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

On 01/25, Christian Brauner wrote:

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When it is reaped is "mostly unrelated".

Then why pidfd_poll() can't simply check !task || task->exit_state ?

Nevermind. So, currently pidfd_poll() succeeds when the leader can be

Hm, the comment right above mentions:

        /*
         * Inform pollers only when the whole thread group exits.
         * If the thread group leader exits before all other threads in the
         * group, then poll(2) should block, similar to the wait(2) family.
         */

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reaped, iow the whole thread group has exited.

Yes, but the comment doesn't contradict with what I have said?

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But even if you are the
parent, you can't expect that wait(WNOHANG) must succeed, the leader
can be traced. I guess it is too late to change this behaviour.

Hm, why is that an issue though?

Well, I didn't say this is a problem. I simply do not know how/why people
use pidfd_poll().

I mostly tried to explain why do I think that do_notify_pidfd() should
be always called from exit_notify() path, not by release_task(), even
if the task is not a leader.

Because a program would rely on WNOHANG to hang on
a ptraced leader? That seems esoteric imho.

To me it would be usefule, but lets not discuss this now. The "patch"
I sent doesn't change the current behaviour.

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What if we add the new PIDFD_THREAD flag? With this flag

	- sys_pidfd_open() doesn't require the must be a group leader

Yes.

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	- pidfd_poll() succeeds when the task passes exit_notify() and
	  becomes a zombie, even if it is a leader and has other threads.

Iiuc, if an existing user creates a pidfd for a thread-group leader and
then polls that pidfd they would currently only get notified if the
thread-group is empty and the leader has exited.

If we now start notifying when the thread-group leader exits but the
thread-group isn't empty then this would be a fairly big api change

Hmm... again, this patch doesn't (shouldn't) change the current behavior.

Please note "with this flag" above. If sys_pidfd_open() was called
without PIDFD_THREAD, then sys_pidfd_open() still requires that the
target task must be a group leader, and pidfd_poll() won't succeed
until the leader exits and thread_group_empty() is true.

Oleg.