linux-kernel - Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

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Date:	Wed, 5 Jan 2011 19:46:32 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Greg KH <greg@...ah.com>
CC:	Jerome Marchand <jmarchan@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

On 2011-01-05 16:58, Greg KH wrote:
> On Wed, Jan 05, 2011 at 02:55:51PM +0100, Jens Axboe wrote:
>> On 2011-01-04 22:00, Greg KH wrote:
>>> On Tue, Jan 04, 2011 at 04:55:13PM +0100, Jerome Marchand wrote:
>>>> Also add a refcount to struct hd_struct to keep the partition in
>>>> memory as long as users exist. We use kref_test_and_get() to ensure
>>>> we don't add a reference to a partition which is going away.
>>>
>>> No, don't do this, use a kref correctly and no such function should be
>>> needed.
>>>
>>>> +	} else {
>>>> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>>>
>>> That is the function that should properly increment the reference count
>>> on the object.  If the object is "being removed", then it will return
>>> NULL and you need to check that.  Do that and you do not need to add:
>>
>> It doesn't matter if you do it in there of after the fact, since the
>> "lock" (RCU) is being held across the call. See my original suggestion
>> here:
>>
>> https://lkml.org/lkml/2010/12/17/275
> 
> Ok, that's fine, just do it without adding that kref function and I have
> no objection :)

Why? The code is perfectly fine. I originally objected to making an API
like this for simple reference counting - seems I was right. Please
actually look at the code and use. Alexey asked whether this was a toy
API or a real one, I'd like to know that as well. If this is meant just
for very basic get/put references, fine, then document that. But then
what's the point of having this API in the first place?

-- 
Jens Axboe

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