pipe->poll_usage was added to ensure that edge-triggered epoll consumers get a wakeup on every write, even if the pipe was already non-empty. However, none of the existing epoll_wakeup_test cases cover this; the test suite passes even with WRITE_ONCE(pipe->poll_usage, true) removed. Add a test that writes twice to a pipe and verifies that epoll_wait with EPOLLET reports data each time. This covers the pipe-specific per-write wakeup behavior that edge-triggered consumers depend on. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Link: https://patch.msgid.link/amnlGZesXu-SUK2H@redhat.com Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>