2022-07-18 03:19:08

by Li Zhijian

Subject: [PATCH] ksefltest: pidfd: Fix wait_states: Test terminated by timeout

0Day/LKP observed that the kselftest blocks forever since one of the
pidfd_wait doesn't terminate in 1 of 30 runs. After digging into
the source, we found that it blocks at:
ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WCONTINUED, NULL), 0);

wait_states has below testing flow:
CHILD PARENT
---------------+--------------
1 STOP itself
2 WAIT for CHILD STOPPED
3 SIGNAL CHILD to CONT
4 CONT
5 STOP itself
5' WAIT for CHILD CONT
6 WAIT for CHILD STOPPED

The problem is that the kernel cannot ensure the order of 5 and 5', once
5's goes first, the test will fail.

we can reproduce it by:
$ while true; do make run_tests -C pidfd; done

Introduce a blocking read in child process to make sure the parent can
check its WCONTINUED.

CC: Philip Li <[email protected]>
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Li Zhijian <[email protected]>
---
I have almost forgotten this patch since the former version post over 6 months
ago. This time I just do a rebase and update the comments.
V2: rewrite with pipe to avoid usleep
---
tools/testing/selftests/pidfd/pidfd_wait.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/tools/testing/selftests/pidfd/pidfd_wait.c b/tools/testing/selftests/pidfd/pidfd_wait.c
index 070c1c876df1..3f7bc6517dea 100644
--- a/tools/testing/selftests/pidfd/pidfd_wait.c
+++ b/tools/testing/selftests/pidfd/pidfd_wait.c
@@ -95,20 +95,27 @@ TEST(wait_states)
.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
.exit_signal = SIGCHLD,
};
+ int ret, pfd[2];
pid_t pid;
siginfo_t info = {
.si_signo = 0,
};

+ ASSERT_EQ(pipe(pfd), 0);
pid = sys_clone3(&args);
ASSERT_GE(pid, 0);

if (pid == 0) {
+ char buf[2];
+ close(pfd[1]);
kill(getpid(), SIGSTOP);
+ ASSERT_EQ(read(pfd[0], buf, 1), 1);
+ close(pfd[0]);
kill(getpid(), SIGSTOP);
exit(EXIT_SUCCESS);
}

+ close(pfd[0]);
ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WSTOPPED, NULL), 0);
ASSERT_EQ(info.si_signo, SIGCHLD);
ASSERT_EQ(info.si_code, CLD_STOPPED);
@@ -117,6 +124,8 @@ TEST(wait_states)
ASSERT_EQ(sys_pidfd_send_signal(pidfd, SIGCONT, NULL, 0), 0);

ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WCONTINUED, NULL), 0);
+ ASSERT_EQ(write(pfd[1], "C", 1), 1);
+ close(pfd[1]);
ASSERT_EQ(info.si_signo, SIGCHLD);
ASSERT_EQ(info.si_code, CLD_CONTINUED);
ASSERT_EQ(info.si_pid, parent_tid);
--
2.36.0

2022-07-19 08:40:20

by Christian Brauner

Subject: Re: [PATCH] ksefltest: pidfd: Fix wait_states: Test terminated by timeout

On Mon, Jul 18, 2022 at 02:58:39AM +0000, [email protected] wrote:
> 0Day/LKP observed that the kselftest blocks forever since one of the
> pidfd_wait doesn't terminate in 1 of 30 runs. After digging into
> the source, we found that it blocks at:
> ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WCONTINUED, NULL), 0);
>
> wait_states has below testing flow:
> CHILD PARENT
> ---------------+--------------
> 1 STOP itself
> 2 WAIT for CHILD STOPPED
> 3 SIGNAL CHILD to CONT
> 4 CONT
> 5 STOP itself
> 5' WAIT for CHILD CONT
> 6 WAIT for CHILD STOPPED
>
> The problem is that the kernel cannot ensure the order of 5 and 5', once
> 5's goes first, the test will fail.
>
> we can reproduce it by:
> $ while true; do make run_tests -C pidfd; done
>
> Introduce a blocking read in child process to make sure the parent can
> check its WCONTINUED.
>
> CC: Philip Li <[email protected]>
> Reported-by: kernel test robot <[email protected]>
> Signed-off-by: Li Zhijian <[email protected]>
> ---
> I have almost forgotten this patch since the former version post over 6 months
> ago. This time I just do a rebase and update the comments.
> V2: rewrite with pipe to avoid usleep
> ---

Thanks for sticking with this!
Reviewed-by: Christian Brauner (Microsoft) <[email protected]>