linux-kernel - Re: [PATCH] ksefltest: pidfd: Fix wait

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Message-ID: <3b2846f3-5d90-afdb-5e18-c30120322ccb@fujitsu.com>
Date:   Mon, 18 Jul 2022 03:08:01 +0000
From:   "lizhijian@...itsu.com" <lizhijian@...itsu.com>
To:     Christian Brauner <brauner@...nel.org>,
        Shuah Khan <shuah@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Philip Li <philip.li@...el.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] ksefltest: pidfd: Fix wait_states: Test terminated by
 timeout



On 18/07/2022 10:58, Li, Zhijian/李 智坚 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.
Correct:
s/once 5's goes first/once 5 goes first


>
> 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>
> ---
> 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);