Hi Grant,
On Nov 15, 2013, at 8:27 AM, Grant Likely wrote:
> On Thu, 14 Nov 2013 10:51:05 +0100, Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>> Hi Grant,
>>
>> On Nov 14, 2013, at 2:44 AM, Grant Likely wrote:
>>
>>> On Wed, 13 Nov 2013 10:03:37 +0100, Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>> On Nov 13, 2013, at 2:34 AM, Grant Likely wrote:
>>>>> On Tue, 12 Nov 2013 11:39:08 +0100, Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>>> On Tue, 5 Nov 2013 19:50:16 +0200, Pantelis Antoniou <panto@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>>>> + } else {
>>>>>>>> + pr_warn("%s: node %p cannot be freed; memory is gone\n",
>>>>>>>> + __func__, node);
>>>>>>>> + }
>>>>>>>> +}
>>>>>>>
>>>>>>> All of the above is potentially dangerous. There is no way to determine
>>>>>>> if anything still holds a reference to a node. The proper way to handle
>>>>>>> removal of properties is to have a release method when the last
>>>>>>> of_node_put is called.
>>>>>>>
>>>>>>
>>>>>> This is safe, and expected to be called only on a dynamically created tree,
>>>>>> that's what all the checks against OF_DYNAMIC guard against.
>>>>>>
>>>>>> It is not ever meant to be called on an arbitrary tree, created by unflattening
>>>>>> a blob.
>>>>>
>>>>> I am talking about when being used on a dynamic tree. The problem is
>>>>> when a driver or other code holds a reference to a dynamic nodes, but
>>>>> doesn't release it correctly. The memory must not be freed until all of
>>>>> the references are relased. OF_DYNAMIC doesn't actually help in that
>>>>> case, and it is the reason for of_node_get()/of_node_put()
>>>>>
>>>>
>>>> I know, but even that is not enough. of_node_get()/of_node_put() handles the
>>>> case of references to the nodes, but not what happens with references to
>>>> properties. deadprops is mitigating the problem somewhat, but if we're going
>>>> to go to all the trouble of kobjectification let's do the props as well.
>>>>
>>>> of_get_property could be modified to return a devm_kmalloced copy of the real
>>>> property and that would deal with most of the callers. Of course for
>>>> the small sized scalar data we can avoid the copy.
>>>>
>>>> By using the devm_* interface we also avoid having to mess too much with the callers.
>>>>
>>>> I.e. what about something like devm_of_get_property()?
>>>
>>> Reference counting is already a horrible pain to keep correct. I don't
>>> see a better way to handle it in the dynamic case, so we're stuck with
>>> it, but I don't want to make it any harder. Adding ref counting to
>>> properties will make it harder than it already is to get the code right.
>>> I'm absolutely fine with a little bit of wasted memory in the form of
>>> deadprops when the alternative is so horrible. References at the node
>>> level is enough granularity.
>>>
>>> I don't think kduping the property is the solution either. I strongly
>>> suspect that will be far more expensive than the deadprop solution.
>>>
>>
>> As long as we can live with deadprops all is fine. Perhaps a devm_of_get_property()
>> makes sense for new drivers though? What do you think? Perhaps copying to a
>> user supplied buffer as well?
>
> I still don't think it is necessary. The device lifetime should always
> be shorter than the node lifetime.
>
>> It's a kind of drag. That means you get handed a device_node pointer you are not
>> able to free it without having the blob along with the container/accessor of it.
>> I.e. For the normal case where the blob comes from a request_firmware() call
>> You have to keep the firmware structure around.
>>
>> Depending on what other method you're going to use tends to make the code a little
>> bit messier.
>
> Understood. Stick with keeping the blob around for now. It can be
> reworkd in the future if necessary since there are no associated
> userspace ABI issues.
>
> g.
OK, will do.
Regards
-- Pantelis--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
