linux-kernel - Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks

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Date:	Tue, 22 Apr 2014 11:34:39 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	rafael.j.wysocki@...el.com, toshi.kani@...com
Subject: Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for
 device online store callbacks

On Mon, 2014-04-21 at 18:46 -0400, Tejun Heo wrote:
> Hello,
> 
> On Mon, Apr 21, 2014 at 05:23:50PM +0800, Li Zhong wrote:
> 
> Proper /** function comment would be nice.

Ok, will try to write some in next version.

> 
> > +struct kernfs_node *lock_device_hotplug_sysfs(struct device *dev,
> > +					       struct device_attribute *attr)
> 
> I can see why you did this but let's please not require the user of
> this function to see how the thing is working internally.  Let's
> return int and keep track of (or look up again) the kernfs_node
> internally.

Ok, it also makes the prototype of lock and unlock look more consistent
and comfortable. 

> 
> >  {
> ...
> > +	/*
> > +	 * We assume device_hotplug_lock must be acquired before removing
> 
> Is this assumption true?  If so, can we add lockdep assertions in
> places to verify and enforce this?  If not, aren't we just feeling
> good when the reality is broken?

It seems not true ... I think there are devices that don't have the
online/offline concept, we just need to add it, remove it, like ethernet
cards. 

Maybe we could change the comments above, like:
	/* We assume device_hotplug_lock must be acquired before 
	 * removing devices, which have online/offline sysfs knob, 
	 * and some locks are needed to serialize the online/offline
	 * callbacks and device removing. ...
? 

And we could add lockdep assertions in cpu and memory related code? e.g.
remove_memory(), unregister_cpu()

Currently, remove_memory() has comments for the function:

 * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
 * and online/offline operations before this call, as required by
 * try_offline_node().
 */
    
maybe it could be removed with the lockdep assertion.

> ...
> 
> Function comment please.

OK. 

Thanks, Zhong

> > +void unlock_device_hotplug_sysfs(struct device *dev,
> > +				 struct kernfs_node *kn)
> > +{
> > +	unlock_device_hotplug();
> > +	kernfs_unbreak_active_protection(kn);
> > +	put_device(dev);
> > +	kernfs_put(kn);
> >  }
> 
> Thanks.
> 


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