Re: [PATCH 08/10] s390/dasd: Display FC Endpoint Security information via sysfs

On 10/7/20 6:40 PM, Cornelia Huck wrote:

On Wed, 7 Oct 2020 16:33:37 +0200
Jan Höppner [off-list ref] wrote:

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+static inline void dasd_path_release(struct kobject *kobj)
+{
+/* Memory for the dasd_path kobject is freed when dasd_free_device() is called */
+}
+    

As already said, I don't think that's a correct way to implement this.
    

As you correctly pointed out, our release function doesn't do anything.
This is because our path data is a (static) part of our device.
This data is critical to keep our devices operational.
We can't simply rely on allocated memory if systems are under stress.   

Yes, avoiding freeing and reallocating memory certainly makes sense.
  

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Having this data dynamically allocated involves a lot of rework of our
path handling as well. There are a few things that are subject to improvement
and evaluating whether our dasd_path structures can be dynamic is one of
these things. However, even then, the above concern persists and I
highly doubt that dynamic dasd_paths objects are doable for us at this
moment.

I do understand the concerns, however, we release the memory for dasd_path
structures eventually when dasd_free_device() is called. Until that point,
the data has to be kept alive. The rest is taking care of by the kobject
library.  

Yes, there doesn't seem to be any memory leakage.
  

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In our path handling we also make sure that we can always verify/validate
paths information even if a system is under high memory pressure. Another
reason why it would contradictory for dasd_path objects to be dynamic.

I hope this explains the reasoning behind the release function.  

I understand where you're coming from.

However, "static" kobjects (in the sense of "we may re-register the
same kobject") are still problematic. Is there any way to simply
"disappear" path objects that are not valid at the moment, or mark them
as not valid?  

You could use kobject_del(), but it is rather intended to be used for
a two-stage removal of the kobject.

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Also, the simple act of registering/unregistering a kobject already
creates stress from its sysfs interactions... it seems you should try
to avoid that as well?
  

We don't re-register kobjects over and over again. The kobjects are
infact initialized and created only _once_. This is done either during
device initialization (after dasd_eckd_read_conf() in
dasd_eckd_check_characteristics()) or when a path is newly added
(in the path event handler).
The kobject will stay until the memory for the whole device is being
freed. This is also the reason why the kobject can stay initialized and
we track ourselves whether we did the initialization/creation already
(which we check e.g. when a path is removed and added again).
So, instead of the release function freeing the kobject data,
it is done by our dasd_free_device() (same thing, different function IMHO).

I think the concerns would be more worrisome if we'd remove/add
the kobjects every time. And then I agree, we'd run into trouble.

The thing that tripped me is

+void dasd_path_remove_kobj(struct dasd_device *device, int chp)
+{
+	if (device->path[chp].in_sysfs) {
+		kobject_put(&device->path[chp].kobj);
+		device->path[chp].in_sysfs = false;
+	}
+}

As an exported function, it is not clear where this may be called from.
Given your explanation above (and some more code reading on my side),
the code looks ok in its current incarnation (but non-idiomatic).

Is there a way to check that indeed nobody re-adds a previously removed
path object due to a (future) programming error? And maybe add a
comment that you must never re-register a path? "The path is gone,
let's remove the object" looks quite tempting.

A comment is the minimum I can think of at the moment and
I'll prepare a fixup patch or a new version of this patch that adds
a proper comment for this function.
Other ways to protect the usage must be investigated. 
I have to discuss with Stefan what the best approach would be as the patchset
is supposed to be ready for upstream integration.

I'd prefer a fixup patch that we could send with at least one more fixup patch
that we have in the pipe already. Let's see. I hope that's fine with you
(and Jens obviously) so far.

regards,
Jan