pidfd: fix a poll race when setting exit_state

pidfd: fix a poll race when setting exit_state

There is a race between reading task->exit_state in pidfd_poll and
writing it after do_notify_parent calls do_notify_pidfd. Expected
sequence of events is:

CPU 0                            CPU 1
------------------------------------------------
exit_notify
  do_notify_parent
    do_notify_pidfd
  tsk->exit_state = EXIT_DEAD
                                  pidfd_poll
                                     if (tsk->exit_state)

However nothing prevents the following sequence:

CPU 0                            CPU 1
------------------------------------------------
exit_notify
  do_notify_parent
    do_notify_pidfd
                                   pidfd_poll
                                      if (tsk->exit_state)
  tsk->exit_state = EXIT_DEAD

This causes a polling task to wait forever, since poll blocks because
exit_state is 0 and the waiting task is not notified again. A stress
test continuously doing pidfd poll and process exits uncovered this bug.

To fix it, we make sure that the task's exit_state is always set before
calling do_notify_pidfd.

Fixes: b53b0b9d9a6 ("pidfd: add polling support")
Cc: [email protected]
Cc: Oleg Nesterov <[email protected]>
Signed-off-by: Suren Baghdasaryan <[email protected]>
Signed-off-by: Joel Fernandes (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[[email protected]: adapt commit message and drop unneeded changes from wait_task_zombie]
Signed-off-by: Christian Brauner <[email protected]>