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

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Date:   Tue, 19 Jul 2022 10:32:29 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     "lizhijian@...itsu.com" <lizhijian@...itsu.com>
Cc:     Shuah Khan <shuah@...nel.org>,
        "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 Mon, Jul 18, 2022 at 02:58:39AM +0000, lizhijian@...itsu.com 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 <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
> ---

Thanks for sticking with this!
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>