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@xxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
Reviewed-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
---
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.
V3: fixes description and add review tag
V2: rewrite with pipe to avoid usleep
---
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 @@ static int sys_waitid(int which, pid_t pid, siginfo_t *info, int options,
.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 @@ static int sys_waitid(int which, pid_t pid, siginfo_t *info, int options,
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);
