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

On 04/29, Joel Fernandes wrote:

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().

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.

Oleg.