Re: [PATCH 1/3] kthread: implement probe_kthread_data()

[Oleg Nesterov <oleg@redhat.com>]

On 03/29, Tejun Heo wrote:
>
> Implement probe_kthread_data() which returns kthread_data if
> accessible.  The function is equivalent to kthread_data() except that
> the specified @task may not be a kthread
                 ^^^^^^^^^^^^^^^^^^^^^^^^^

This doesn't necessarily mean its ->vfork_done != NULL... but I guess
the "false positive" is fine for your purpose.

> or its vfork_done is already
> cleared rendering struct kthread inaccessible.
> ...

> +/**
> + * probe_kthread_data - speculatively version of kthread_data()
> + * @task: possible kthread task in question
> + *
> + * @task could be a kthread task.  Return the data value specified when it
> + * was created if accessible.  If @task isn't a kthread task or its data is
> + * inaccessible for any reason, %NULL is returned.  This function requires
> + * that @task itself is safe to dereference.
> + */
> +void *probe_kthread_data(struct task_struct *task)
> +{
> +	struct kthread *kthread = to_kthread(task);
> +	void *data = NULL;
> +
> +	probe_kernel_read(&data, &kthread->data, sizeof(data));
> +	return data;
> +}

OK, but we can simply check ->vfork_done != NULL ?

We do not care if we race with the exiting thread which can clear
its ->vfork_done. If it is safe to dereference this @task then
kthread is also safe.

kthread-introduce-to_live_kthread.patch in -mm adds the trivial
helper,

	static inline struct kthread *to_kthread(struct task_struct *k)
	{
	       return __to_kthread(k->vfork_done);
	}

	static struct kthread *to_live_kthread(struct task_struct *k)
	{
	       struct completion *vfork = ACCESS_ONCE(k->vfork_done);
	       if (likely(vfork))
		       return __to_kthread(vfork);
	       return NULL;
	}

So, perhaps,

	voif *kthread_data_safe(struct task_struct *task)
	{
		struct kthread *kthread = to_live_kthread(task);
		return kthread ? kthread->data : NULL;
	}

?

Oleg.