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



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