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

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Message-ID: <4D247984.2030103@fusionio.com>
Date:	Wed, 5 Jan 2011 15:00:36 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Jerome Marchand <jmarchan@...hat.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 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(). As per my last email, you
need a check in the rcu callback to ensure that the ref is 0.


-- 
Jens Axboe

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