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

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Date:	Thu, 14 Oct 2010 16:30:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jens Axboe <axboe@...nel.dk>
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 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.

							Thanx, Paul
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