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

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Message-ID: <4D249A07.5020507@redhat.com>
Date:	Wed, 05 Jan 2011 17:19:19 +0100
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Greg KH <greg@...ah.com>
CC:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <jaxboe@...ionio.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 01/05/2011 05:00 PM, Greg KH wrote:
> On Wed, Jan 05, 2011 at 02:51:28PM +0100, Jerome Marchand wrote:
>> On 01/04/2011 10:00 PM, 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.
>>
>> Agreed.
>>
>>>  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:
>>
>> The object is actually removed in a rcu callback function. We could
>> certainly add a flag to hd_struct, set by the release function, to
>> indicate disk_map_sector_rcu() that the partition is being removed, but
>> why not use the refcount instead?
> 
> Because you have to properly serialize the grabbing of a kref if you
> don't have a valid pointer in the first place, otherwise it will not
> work properly at all.  Your new function still does not properly handle
> the race condition of dropping the last reference and then having the
> kref be cleaned up.  You are giving false hope to the user of the api
> that what they are doing is correct.
> 

For clarification, is your objection only about not adding that misleading
function to kref api (I understand that), or is my code actually racy?

thanks,
Jerome

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