Re: [PATCH] blk: fix a wrong accounting of hd_struct->in_flight

Jens Axboe
2010-10-15T07:30:11Z

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