Re: [PATCH v1 1/2] Add polling support to pidfd

On Mon, Apr 29, 2019 at 04:20:30PM +0200, Oleg Nesterov wrote:

On 04/29, Joel Fernandes wrote:

quoted

However, in your code above, it is avoided because we get:

Task A (poller)		Task B (exiting task being polled)
------------            ----------------
poll() called
add_wait_queue()
			exit_state is set to non-zero
read exit_state
remove_wait_queue()
			wake_up_all()

just to clarify... No, sys_poll() path doesn't do remove_wait_queue() until
it returns to user mode, and that is why we can't race with set-exit_code +
wake_up().

I didn't follow what you mean, the removal from the waitqueue happens in
free_poll_entry() called from poll_freewait() which happens from
do_sys_poll() which is before the syscall returns to user mode. Could you
explain more?

pidfd_poll() can race with the exiting task, miss exit_code != 0, and return
zero. However, do_poll() won't block after that and pidfd_poll() will be called
again.

Here also I didn't follow what you mean. If exit_code is read as 0 in
pidfd_poll(), then in do_poll() the count will be 0 and it will block in
poll_schedule_timeout(). Right? But above you're saying it wont block.
Also if you could show a timing diagram of this different race you're talking
about, that will make things clear. It is a bit hard for me to picture
otherwise.

Also, I will use task_pid() for getting the pid from the task, as you suggest
in the other thread.

thanks,

- Joel