linux-kernel - Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd

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Message-ID: <20200703203021.paebx25miovmaxqt@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 3 Jul 2020 13:30:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of
 pidfd_poll

On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote:
> Create an independent helper thread_group_exited report return true
> when all threads have passed exit_notify in do_exit.  AKA all of the
> threads are at least zombies and might be dead or completely gone.
> 
> Create this helper by taking the logic out of pidfd_poll where
> it is already tested, and adding a missing READ_ONCE on
> the read of task->exit_state.
> 
> I will be changing the user mode driver code to use this same logic
> to know when a user mode driver needs to be restarted.
> 
> Place the new helper thread_group_exited in kernel/exit.c and
> EXPORT it so it can be used by modules.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  include/linux/sched/signal.h |  2 ++
>  kernel/exit.c                | 24 ++++++++++++++++++++++++
>  kernel/fork.c                |  6 +-----
>  3 files changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 0ee5e696c5d8..1bad18a1d8ba 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -674,6 +674,8 @@ static inline int thread_group_empty(struct task_struct *p)
>  #define delay_group_leader(p) \
>  		(thread_group_leader(p) && !thread_group_empty(p))
>  
> +extern bool thread_group_exited(struct pid *pid);
> +
>  extern struct sighand_struct *__lock_task_sighand(struct task_struct *task,
>  							unsigned long *flags);
>  
> diff --git a/kernel/exit.c b/kernel/exit.c
> index d3294b611df1..a7f112feb0f6 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1713,6 +1713,30 @@ COMPAT_SYSCALL_DEFINE5(waitid,
>  }
>  #endif
>  
> +/**
> + * thread_group_exited - check that a thread group has exited
> + * @pid: tgid of thread group to be checked.
> + *
> + * Test if thread group is has exited (all threads are zombies, dead
> + * or completely gone).
> + *
> + * Return: true if the thread group has exited. false otherwise.
> + */
> +bool thread_group_exited(struct pid *pid)
> +{
> +	struct task_struct *task;
> +	bool exited;
> +
> +	rcu_read_lock();
> +	task = pid_task(pid, PIDTYPE_PID);
> +	exited = !task ||
> +		(READ_ONCE(task->exit_state) && thread_group_empty(task));
> +	rcu_read_unlock();
> +
> +	return exited;
> +}

I'm not sure why you think READ_ONCE was missing.
It's different in wait_consider_task() where READ_ONCE is needed because
of multiple checks. Here it's done once.

The rest all looks good to me. Tested with and without bpf_preload patches.
Feel free to create a frozen branch with this set.

btw I'll be offline starting tomorrow for a week.
Will catch up with threads afterwards.