Re: [PATCH 8/19]: SCST SYSFS interface implementation

[Joe Eykholt <jeykholt@cisco.com>]

On 11/10/10 12:19 PM, Vladislav Bolkhovitin wrote:
> Boaz Harrosh, on 11/10/2010 12:58 PM wrote:
>> On 11/09/2010 10:06 PM, Vladislav Bolkhovitin wrote:
>>>
>>> Sorry, but what is incorrect in the working implementation without any
>>> bugs doing its job in the simplest, smallest and clearest way?
>>>
>>> If those objects remade to free themselves in the kobjects release(),
>>> what value would it add to them? Would the implementation be simpler,
>>> smaller or clearer? Not, I believe, new implementation would be only
>>> bigger and less clear. So, what's the point to do it to make the code worse?
>>>
>>
>> Totally theoretically speaking, since I have not inspected the code.
>>
>> If today you wait for the count to reach zero, then unregister
>> and send an event to some other subsystem to free the object.
>>
>> Is it not the same as if you take an extra refcount, unregister and
>> send the event at count=1. Then at that other place decrement the last
>> count to cause the object to be freed.
>>
>> I agree that it is hard to do lockless. what some places do is have
>> an extra kref. The kobj has a single ref on it. everything takes the
>> other kref. when that reaches zero the unregister and event fires
>> and at free you decrement the only kobj ref to deallocate. This is one
>> way. In some situations you can manage with a single counter it all
>> depends.
> 
> Thanks for sharing your thoughts with us. But the question isn't about
> if it's possible to implement what we need locklessly. The question is
> in two approaches how to synchronously delete objects with entries on SYSFS:
> 
> 1. struct object_x {
> 	...
> 	struct kobject kobj;
> 	struct completion *release_completion;
> };
> 
> static void x_release(struct kobject *kobj)
> {
> 	struct object_x *x;
> 	struct completion *c;
> 
> 	x = container_of(kobj, struct object_x, kobj);
> 	c = x->release_completion;
> 	kfree(x);
> 	complete_all(c);
> }
> 
> void del_object(struct object_x *x)
> {
> 	DECLARE_COMPLETION_ONSTACK(completion);
> 
> 	...
> 	x->release_completion = &completion;
> 	kobject_put(&x->kobj);
> 	wait_for_completion(&completion);
> }
> 
> and
> 
> 2. struct object_x {
> 	...
> 	struct kobject kobj;
> 	struct completion release_completion;
> };
> 
> static void x_release(struct kobject *kobj)
> {
> 	struct object_x *x;
> 
> 	x = container_of(kobj, struct object_x, kobj);
> 	complete_all(&x->release_completion);
> }
> 
> void del_object(struct object_x *x)
> {
> 	...
> 	kobject_put(&x->kobj);
> 	wait_for_completion(&completion);
> 	...
> 	kfree(x);
> }

I'll admit I don't understand this all that well, but
why not just have x_release() (based on (2))
do free(x), and have del_object
do the kobject_put(&x->kobj) as its very last thing?
Then you don't need the completion.

> 
> Greg asserts that (1) is the only correct approach while (2) is
> incorrect, and I'm trying to justify that (2) is correct too and
> sometimes could be better, i.e. simpler and clearer, because it
> decouples object_x from SYSFS and its kobj. Then kobj becomes an
> ordinary member of struct object_x without any special treatment and
> with the same lifetime rules as other members of struct object_x. While
> in (1) all lifetime of struct object_x is strictly attached to kobj, so
> it needs be specially handled with additional code for that if struct
> object_x has many other members which needed to be initialized/deleted
> _before and after_ kobj as we have in SCST.
> 
> Vlad