kthread: save thread function

It's handy to keep the kthread_fn just as a unique cookie to identify
classes of kthreads.  E.g. if you can verify that a given task is
running your thread_fn, then you may know what sort of type kthread_data
points to.

We'll use this in nfsd to pass some information into the vfs.  Note it
will need kthread_data() exported too.
Original-patch-by: Tejun Heo <tj@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/kthread.h

@@ -57,6 +57,7 @@ bool kthread_should_stop(void);
 bool kthread_should_park(void);
 bool __kthread_should_park(struct task_struct *k);
 bool kthread_freezable_should_stop(bool *was_frozen);
+void *kthread_func(struct task_struct *k);
 void *kthread_data(struct task_struct *k);
 void *kthread_probe_data(struct task_struct *k);
 int kthread_park(struct task_struct *k);

kernel/kthread.c

@@ -46,6 +46,7 @@ struct kthread_create_info
 struct kthread {
 	unsigned long flags;
 	unsigned int cpu;
+	int (*threadfn)(void *);
 	void *data;
 	struct completion parked;
 	struct completion exited;
@@ -152,6 +153,20 @@ bool kthread_freezable_should_stop(bool *was_frozen)
 }
 EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
 
+/**
+ * kthread_func - return the function specified on kthread creation
+ * @task: kthread task in question
+ *
+ * Returns NULL if the task is not a kthread.
+ */
+void *kthread_func(struct task_struct *task)
+{
+	if (task->flags & PF_KTHREAD)
+		return to_kthread(task)->threadfn;
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(kthread_func);
+
 /**
  * kthread_data - return data value specified on kthread creation
  * @task: kthread task in question
@@ -164,6 +179,7 @@ void *kthread_data(struct task_struct *task)
 {
 	return to_kthread(task)->data;
 }
+EXPORT_SYMBOL_GPL(kthread_data);
 
 /**
  * kthread_probe_data - speculative version of kthread_data()
@@ -244,6 +260,7 @@ static int kthread(void *_create)
 		do_exit(-ENOMEM);
 	}
 
+	self->threadfn = threadfn;
 	self->data = data;
 	init_completion(&self->exited);
 	init_completion(&self->parked);