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    SubjectRe: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks
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    On Tue, 2014-04-22 at 12:11 +0200, Rafael J. Wysocki wrote:
    > On Tuesday, April 22, 2014 11:34:39 AM Li Zhong wrote:
    > > On Mon, 2014-04-21 at 18:46 -0400, Tejun Heo wrote:
    > > > Hello,
    > > >
    > > > On Mon, Apr 21, 2014 at 05:23:50PM +0800, Li Zhong wrote:
    > > >
    > > > Proper /** function comment would be nice.
    > >
    > > Ok, will try to write some in next version.
    > >
    > > >
    > > > > +struct kernfs_node *lock_device_hotplug_sysfs(struct device *dev,
    > > > > + struct device_attribute *attr)
    > > >
    > > > I can see why you did this but let's please not require the user of
    > > > this function to see how the thing is working internally. Let's
    > > > return int and keep track of (or look up again) the kernfs_node
    > > > internally.
    > >
    > > Ok, it also makes the prototype of lock and unlock look more consistent
    > > and comfortable.
    > >
    > > >
    > > > > {
    > > > ...
    > > > > + /*
    > > > > + * We assume device_hotplug_lock must be acquired before removing
    > > >
    > > > 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.
    >
    > Well, I haven't been following this closely (I was travelling, sorry), but
    > there certainly are devices without online/offline. That currently is only
    > present for CPUs, memory blocks and ACPI containers (if I remember correctly).
    >
    > >
    > > 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. ...
    > > ?
    >
    > Lockdep assertions would be better than this in my opinion.

    This is talking about the lock required in the other process, the device
    removing process, e.g. that in remove_memory() below. So I guess no
    lockdep assertions needed here. Or I misunderstand your point?

    > >
    > > 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.
    >
    > No, please don't remove it. It is there to explain where the locking requirement
    > comes from.

    OK, I see. I think I'll just add lockdep assertions, and keep the
    comments there.

    Thanks, Zhong

    >




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