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

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Message-ID: <1398229422.2805.49.camel@ThinkPad-T5421.cn.ibm.com>
Date:	Wed, 23 Apr 2014 13:03:42 +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 Tue, 2014-04-22 at 16:44 -0400, Tejun Heo wrote:
> Hello,
> 
> On Tue, Apr 22, 2014 at 11:34:39AM +0800, Li Zhong wrote:
> > > 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.
> 
> I'm confused about the overall locking scheme.  What's the role of
> device_hotplug_lock?  Is that solely to prevent the sysfs deadlock
> issue?  Or does it serve other synchronization purposes depending on
> the specific subsystem?  If the former, the lock no longer needs to
> exist.  The only thing necessary would be synchronization between
> device_del() deleting the sysfs file and the unbreak helper invoking
> device-specific callback.  If the latter, we probably should change
> that.  Sharing hotplug lock across multiple subsystems through driver
> core sounds like a pretty bad idea.

I think it's the latter. I think device_{on|off}line is better to be
done in some sort of lock which prevents the device from being removed,
including some preparation work that needs be done before device_del().

Thanks, Zhong

> 
> Thanks.
> 


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