Messages in this thread | | | Date | Wed, 23 Apr 2014 00:21:33 +0200 | From | "Rafael J. Wysocki" <> | Subject | Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks |
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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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