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

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Message-ID: <4D247B93.2010900@redhat.com>
Date:	Wed, 05 Jan 2011 15:09:23 +0100
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
CC:	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>,
	Greg Kroah-Hartman <greg@...ah.com>
Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges

On 01/05/2011 03:00 PM, Jens Axboe wrote:
> On 2011-01-04 16:55, Jerome Marchand wrote:
>> +	} else {
>> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>> +		if (!kref_test_and_get(&part->ref)) {
>> +			/*
>> +			 * The partition is already being removed,
>> +			 * the request will be accounted on the disk only
>> +			 *
>> +			 * We take a reference on disk->part0 although that
>> +			 * partition will never be deleted, so we can treat
>> +			 * it as any other partition.
>> +			 */
>> +			part = &rq->rq_disk->part0;
>> +			kref_get(&part->ref);
>> +		}
> 
> This still doesn't work. So you are inside the if {} block, you know
> what someone has dropped the last reference and the call_rcu() is being
> scheduled. Adding a reference now will not prevent 'part' from going
> away as soon as you do part_stat_unlock().

And what is the problem with that since we don't use 'part' (as returned
by disk_map_sector_rcu()), but disk->part0 instead?

> As per my last email, you
> need a check in the rcu callback to ensure that the ref is 0.
> 
> 

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