linux-kernel - [PATCH AUTOSEL 6.0 09/30] ksefltests: pidfd: Fix wait

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Date:   Thu, 10 Nov 2022 21:33:17 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Li Zhijian <lizhijian@...itsu.com>,
        Philip Li <philip.li@...el.com>,
        kernel test robot <lkp@...el.com>,
        Christian Brauner <brauner@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>, shuah@...nel.org,
        linux-kselftest@...r.kernel.org
Subject: [PATCH AUTOSEL 6.0 09/30] ksefltests: pidfd: Fix wait_states: Test terminated by timeout

From: Li Zhijian <lizhijian@...itsu.com>

[ Upstream commit 88e1f16ba58665e9edfce437ea487da2fa759af9 ]

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 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 <philip.li@...el.com>
Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Li Zhijian <lizhijian@...itsu.com>
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>
Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/testing/selftests/pidfd/pidfd_wait.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/pidfd/pidfd_wait.c b/tools/testing/selftests/pidfd/pidfd_wait.c
index 070c1c876df1..c3e2a3041f55 100644
--- a/tools/testing/selftests/pidfd/pidfd_wait.c
+++ b/tools/testing/selftests/pidfd/pidfd_wait.c
@@ -95,20 +95,28 @@ TEST(wait_states)
 		.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
 		.exit_signal = SIGCHLD,
 	};
+	int 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 +125,8 @@ 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);
-- 
2.35.1