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

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Message-ID: <10378860.X4mJXjBVbJ@vostro.rjw.lan>
Date:	Wed, 23 Apr 2014 12:58:40 +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 Wednesday, April 23, 2014 01:03:42 PM Li Zhong wrote:
> 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.

Actually, no, this is not the case if I understand you correctly.

> 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().

Quite frankly, you should be confident that you understand the code you're
trying to modify or please don't touch it.

I'll have a deeper look at this issue later today or tomorrow and will get
back to you then.

Thanks!

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