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.
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