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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