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

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Date:	Sun, 22 Sep 2013 13:04:17 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Tomoki Sekiyama <tomoki.sekiyama@....com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk,
	seiji.aguchi@....com, vgoyal@...hat.com, majianpeng@...il.com
Subject: Re: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md
 device initialization

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

Thanks.

-- 
tejun
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