linux-kernel - Re: [PATCH 0/4] allow multiple kthreadd's

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Date:   Wed, 6 May 2020 11:36:58 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        Jeff Layton <jlayton@...hat.com>,
        David Howells <dhowells@...hat.com>, Shaohua Li <shli@...com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] allow multiple kthreadd's

On Tue, May 05, 2020 at 05:25:27PM -0400, J. Bruce Fields wrote:
> On Tue, May 05, 2020 at 05:09:56PM -0400, Tejun Heo wrote:
> > It's not the end of the world but a bit hacky. I wonder whether something
> > like the following would work better for identifying worker type so that you
> > can do sth like
> > 
> >  if (kthread_fn(current) == nfsd)
> >         return kthread_data(current);
> >  else
> >         return NULL;     
> 
> Yes, definitely more generic, looks good to me.

This is what I'm testing with.

If it's OK with you, could I add your Signed-off-by and take it through
the nfsd tree? I'll have some other patches that will depend on it.

--b.


commit 379bfe5257b6
Author: Tejun Heo <tj@...nel.org>
Date:   Tue May 5 21:26:07 2020 -0400

    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.
    
    Signed-off-by: J. Bruce Fields <bfields@...hat.com>

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 8bbcaad7ef0f..c00ee443833f 100644
--- a/include/linux/kthread.h
+++ b/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_fn(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);
diff --git a/kernel/kthread.c b/kernel/kthread.c
index bfbfa481be3a..b87c4a9ba91d 100644
--- a/kernel/kthread.c
+++ b/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_fn - return the function specified on kthread creation
+ * @task: kthread task in question
+ *
+ * Returns NULL if the task is not a kthread.
+ */
+void *kthread_fn(struct task_struct *task)
+{
+	if (task->flags & PF_KTHREAD)
+		return to_kthread(task)->threadfn;
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(kthread_fn);
+
 /**
  * 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);