linux-kernel - Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization

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Date:	Mon, 23 Sep 2013 20:11:55 +0000
From:	Tomoki Sekiyama <tomoki.sekiyama@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Seiji Aguchi <seiji.aguchi@....com>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"majianpeng@...il.com" <majianpeng@...il.com>
Subject: Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and
 md device initialization

Hi Tejun,

Thank you for the review.

On 9/22/13 13:04 , "Tejun Heo" <tj@...nel.org> wrote:

>On Fri, Aug 30, 2013 at 06:47:07PM -0400, Tomoki Sekiyama wrote:
>> @@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q,
>>request_fn_proc *rfn,
>>  
>>  	q->sg_reserved_size = INT_MAX;
>>  
>> +	/* Protect q->elevator from elevator_change */
>> +	mutex_lock(&q->sysfs_lock);
>> +
>>  	/* init elevator */
>> -	if (elevator_init(q, NULL))
>> +	if (elevator_init(q, NULL)) {
>> +		mutex_unlock(&q->sysfs_lock);
>>  		return NULL;
>> +	}
>> +
>> +	mutex_unlock(&q->sysfs_lock);
>> +
>>  	return q;
>>  }
>>  EXPORT_SYMBOL(blk_init_allocated_queue);
>> diff --git a/block/elevator.c b/block/elevator.c
>> index 668394d..02d4390 100644
>> --- a/block/elevator.c
>> +++ b/block/elevator.c
>> @@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char
>>*name)
>>  	struct elevator_type *e = NULL;
>>  	int err;
>>  
>> +	/*
>> +	 * q->sysfs_lock must be held to provide mutual exclusion between
>> +	 * elevator_switch() and here.
>> +	 */
>> +	lockdep_assert_held(&q->sysfs_lock);
>> +
>>  	if (unlikely(q->elevator))
>>  		return 0;
>
>Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
>where necessary?  

The locking cannot be moved into elevator_init() because it is called
from elevator_switch() path, where the request_queue's sysfs_lock is
already taken.

> It'd be more consistent with elevator_exit() that way.

What elevator_exit() locks is elevator_queue's sysfs_lock, not
request_queue's sysfs_lock. What we need here is request_queue's
sysfs_lock.

>Thanks.
>
>-- 
>Tejun


Thanks,
Tomoki Sekiyama


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