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 12:11:53 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	Tejun Heo <tj@...nel.org>, 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 Tuesday, April 22, 2014 11:34:39 AM Li Zhong wrote:
> 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. 

Well, I haven't been following this closely (I was travelling, sorry), but
there certainly are devices without online/offline.  That currently is only
present for CPUs, memory blocks and ACPI containers (if I remember correctly).

> 
> 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. ...
> ? 

Lockdep assertions would be better than this in my opinion.

> 
> 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.

No, please don't remove it.  It is there to explain where the locking requirement
comes from.

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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