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

[Jerome Marchand]

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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