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

[Rafael J. Wysocki]

On 4/22/2014 10:44 PM, 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.

Can you please elaborate a bit?

It is there to protect hotplug operations involving multiple devices (in 
different subsystems) from racing with each other.  Why exactly is it bad?

Rafael

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/