linux-kernel - Re: [PATCH] blk: fix a wrong accounting of hd_struct->in

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Date:	Fri, 15 Oct 2010 09:30:06 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	paulmck@...ux.vnet.ibm.com
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk: fix a wrong accounting of hd_struct->in_flight

On 2010-10-15 01:30, Paul E. McKenney wrote:
> On Thu, Oct 14, 2010 at 02:44:32PM +0200, Jens Axboe wrote:
>> On 2010-10-14 08:07, KOSAKI Motohiro wrote:
>>>> @@ -1268,7 +1270,17 @@ static int __make_request(struct request
>>>>  		 * not touch req->buffer either...
>>>>  		 */
>>>>  		req->buffer = bio_data(bio);
>>>> +		src_part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
>>>>  		req->__sector = bio->bi_sector;
>>>> +		dst_part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
>>>
>>> I think this is wrong. disk_map_sector_rcu() require
>>> rcu read lock held (see function comment). all other call site take 
>>> part_stat_lock() before disk_map_sector_rcu() call.
>>
>> It's called under the queue lock with irqs disabled, which implies a
>> rcu critical section.
> 
> Having irqs disabled does imply an rcu_read_lock_sched() or an
> rcu_read_lock_bh(), but not an rcu_read_lock(), especially if
> CONFIG_PREEMPT_RCU.
> 
> So an explicit rcu_read_lock() does seem to be needed here.

Thanks Paul, I stand corrected. The final patch will be vastly
different, but it's surely worth keeping in mind.


-- 
Jens Axboe

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