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

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Message-ID: <4D247867.9040502@fusionio.com>
Date:	Wed, 5 Jan 2011 14:55:51 +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 2011-01-04 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

-- 
Jens Axboe

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