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    SubjectRe: [PATCH v4] Drivers: hv: vmbus: Expose monitor data only when monitor pages are used
    On Sun, Mar 03, 2019 at 04:11:28PM -0500, Kimberly Brown wrote:
    > On Sun, Mar 03, 2019 at 09:05:43AM +0100, Greg KH wrote:
    > > On Fri, Mar 01, 2019 at 02:18:24PM -0500, Kimberly Brown wrote:
    > > > +/*
    > > > + * Channel-level attribute_group callback function. Returns the permission for
    > > > + * each attribute, and returns 0 if an attribute is not visible.
    > > > + */
    > > > +static umode_t vmbus_chan_attr_is_visible(struct kobject *kobj,
    > > > + struct attribute *attr, int idx)
    > > > +{
    > > > + const struct vmbus_channel *channel =
    > > > + container_of(kobj, struct vmbus_channel, kobj);
    > > > +
    > > > + /* Hide the monitor attributes if the monitor mechanism is not used. */
    > > > + if (!channel->offermsg.monitor_allocated &&
    > > > + (attr == &chan_attr_pending.attr ||
    > > > + attr == &chan_attr_latency.attr ||
    > > > + attr == &chan_attr_monitor_id.attr))
    > > > + return 0;
    > > > +
    > > > + return attr->mode;
    > > > +}
    > > > +
    > > > +static struct attribute_group vmbus_chan_group = {
    > > > + .attrs = vmbus_chan_attrs,
    > > > + .is_visible = vmbus_chan_attr_is_visible
    > > > +};
    > > > +
    > > > static struct kobj_type vmbus_chan_ktype = {
    > > > .sysfs_ops = &vmbus_chan_sysfs_ops,
    > > > .release = vmbus_chan_release,
    > > > - .default_attrs = vmbus_chan_attrs,
    > >
    > > Why did you remove this line?
    >
    > I removed the default attributes because vmbus_chan_attrs contains
    > non-default attributes. You suggested that I use one attribute_group and
    > an is_visible() callback for the device-level attributes (see
    > https://lore.kernel.org/lkml/20190226081848.GA15659@kroah.com/). I
    > assumed (possibly incorrectly) that I should do the same for these
    > channel-level attributes.

    That is fine to have a callback, but why did you have to remove the
    default attribute pointer? The two should have nothing to do with each
    other.

    > > > };
    > > >
    > > > /*
    > > > @@ -1571,6 +1624,12 @@ int vmbus_add_channel_kobj(struct hv_device *dev, struct vmbus_channel *channel)
    > > > if (ret)
    > > > return ret;
    > > >
    > > > + ret = sysfs_create_group(kobj, &vmbus_chan_group);
    > >
    > > Why are you adding these "by hand"? What was wrong with using the
    > > default attribute group pointer? You also are not removing the
    > > attributes :(
    >
    > Are you referring to default_attrs in kobj_type? It's not an
    > attribute_group pointer, it's a pointer to an attribute pointer array.
    > The problem with using default_attrs is that all of the attributes are
    > visible.

    It shouldn't, the is_visable() callback will be called on each attribute
    when the group is created by the core. Did that not work properly when
    you tested this?

    > I'm fairly certain that the monitor attributes are being removed.
    > sysfs_create_group() uses the attribute_group's is_visible() callback to
    > control the attribute visibility. And, when I look at the sysfs files, I
    > can see that the monitor sysyfs files are removed.

    I mean you are not removing the group when the device goes away, not
    that the individual files are not present. If you leave the pointer to
    default_attributes there, the core will properly remove the sysfs
    attributes when the device is removed from the system. Otherwise you
    just "get lucky" if the attributes are removed or not.

    > In v3, I proposed moving the monitor attributes to a special
    > attribute_group and adding that group manually when needed. Do you
    > prefer that approach for the channel-level monitor attributes?

    No need for a special group here, just use the is_visable() callback
    like you currently are, all should be fine. I think you are adding more
    code than you need to in order to get this to all work properly :)

    thanks,

    greg k-h

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