[PATCH v2] selftests/epoll: add a regression test for pipe->poll_usage

[Oleg Nesterov <oleg@redhat.com>]

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>
---
 .../filesystems/epoll/epoll_wakeup_test.c     | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/testing/selftests/filesystems/epoll/epoll_wakeup_test.c b/tools/testing/selftests/filesystems/epoll/epoll_wakeup_test.c
index f6f1a7ff01b0..81a994943e12 100644
--- a/tools/testing/selftests/filesystems/epoll/epoll_wakeup_test.c
+++ b/tools/testing/selftests/filesystems/epoll/epoll_wakeup_test.c
@@ -3538,4 +3538,27 @@ TEST(epoll65)
 	close(ctx.efd[1]);
 }
 
+TEST(epoll66)
+{
+	struct epoll_event event;
+	int pfd[2], efd;
+
+	ASSERT_EQ(pipe(pfd), 0);
+
+	efd = epoll_create1(0);
+	ASSERT_GE(efd, 0);
+
+	event.events = EPOLLIN | EPOLLET;
+	ASSERT_EQ(epoll_ctl(efd, EPOLL_CTL_ADD, pfd[0], &event), 0);
+
+	for (int i = 0; i < 2; ++i) {
+		ASSERT_EQ(write(pfd[1], "", 1), 1);
+		EXPECT_EQ(epoll_wait(efd, &event, 1, 0), 1);
+	}
+
+	close(pfd[0]);
+	close(pfd[1]);
+	close(efd);
+}
+
 TEST_HARNESS_MAIN
-- 
2.52.0