Re: [PATCH v3 13/16] exit: Factor thread_group_exited out of pidfd_poll

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Jul 02, 2020 at 11:41:37AM -0500, Eric W. Biederman wrote:
> Create an independent helper thread_group_exited report return true

s/report return/which reports/

> 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 would prefer to have this comment dropped as this read_once() is not
missing as you can see from the comments elsewhere in this thread.

> 
> 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@xmission.com>
> ---

Minus the typos above and below, this looks good and passes the pidfd
and process test-suite.
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

Thanks!
Christian

>  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

s/is has exited/has exited/

> + * 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;
> +}
> +EXPORT_SYMBOL(thread_group_exited);
> +
>  __weak void abort(void)
>  {
>  	BUG();
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 142b23645d82..bf215af7a904 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1787,22 +1787,18 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
>   */
>  static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
>  {
> -	struct task_struct *task;
>  	struct pid *pid = file->private_data;
>  	__poll_t poll_flags = 0;
>  
>  	poll_wait(file, &pid->wait_pidfd, pts);
>  
> -	rcu_read_lock();
> -	task = pid_task(pid, PIDTYPE_PID);
>  	/*
>  	 * Inform pollers only when the whole thread group exits.
>  	 * If the thread group leader exits before all other threads in the
>  	 * group, then poll(2) should block, similar to the wait(2) family.
>  	 */
> -	if (!task || (task->exit_state && thread_group_empty(task)))
> +	if (thread_group_exited(pid))
>  		poll_flags = EPOLLIN | EPOLLRDNORM;
> -	rcu_read_unlock();
>  
>  	return poll_flags;
>  }
> -- 
> 2.25.0
>