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

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Date:	Wed, 10 Nov 2010 12:42:01 -0800
From:	Joe Eykholt <jeykholt@...co.com>
To:	Vladislav Bolkhovitin <vst@...b.net>
CC:	Boaz Harrosh <bharrosh@...asas.com>, Greg KH <greg@...ah.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	scst-devel <scst-devel@...ts.sourceforge.net>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Vu Pham <vuhuong@...lanox.com>,
	Bart Van Assche <bart.vanassche@...il.com>,
	James Smart <James.Smart@...lex.Com>,
	Andy Yan <ayan@...vell.com>,
	Chetan Loke <generationgnu@...oo.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Hannes Reinecke <hare@...e.de>,
	Richard Sharpe <realrichardsharpe@...il.com>,
	Daniel Henrique Debonzi <debonzi@...ux.vnet.ibm.com>
Subject: Re: [PATCH 8/19]: SCST SYSFS interface implementation



On 11/10/10 12:29 PM, Joe Eykholt wrote:
> 
> 
> 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.

Ah, well to answer my own question, I guess that's (1).
Never mind.

	Joe

>> 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
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