Re: [PATCH 20/24] block: RCU free request_queue

[Vivek Goyal]

On Fri, Dec 28, 2012 at 12:35:42PM -0800, Tejun Heo wrote:
> RCU free request_queue so that blkcg_gq->q can be dereferenced under
> RCU lock.  This will be used to implement hierarchical stats.

Can we just take a blkg reference on ->q in blkg_alloc() and drop that
reference in blkg_free(), instead of RCU freeing up queue.

Thanks
Vivek

> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  block/blk-sysfs.c      | 9 ++++++++-
>  include/linux/blkdev.h | 2 ++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 7881477..6206a93 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -497,6 +497,13 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
>  	return res;
>  }
>  
> +static void blk_free_queue_rcu(struct rcu_head *rcu_head)
> +{
> +	struct request_queue *q = container_of(rcu_head, struct request_queue,
> +					       rcu_head);
> +	kmem_cache_free(blk_requestq_cachep, q);
> +}
> +
>  /**
>   * blk_release_queue: - release a &struct request_queue when it is no longer needed
>   * @kobj:    the kobj belonging to the request queue to be released
> @@ -538,7 +545,7 @@ static void blk_release_queue(struct kobject *kobj)
>  	bdi_destroy(&q->backing_dev_info);
>  
>  	ida_simple_remove(&blk_queue_ida, q->id);
> -	kmem_cache_free(blk_requestq_cachep, q);
> +	call_rcu(&q->rcu_head, blk_free_queue_rcu);
>  }
>  
>  static const struct sysfs_ops queue_sysfs_ops = {
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index f94bc83..406343c 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -19,6 +19,7 @@
>  #include <linux/gfp.h>
>  #include <linux/bsg.h>
>  #include <linux/smp.h>
> +#include <linux/rcupdate.h>
>  
>  #include <asm/scatterlist.h>
>  
> @@ -437,6 +438,7 @@ struct request_queue {
>  	/* Throttle data */
>  	struct throtl_data *td;
>  #endif
> +	struct rcu_head		rcu_head;
>  };
>  
>  #define QUEUE_FLAG_QUEUED	1	/* uses generic tag queueing */
> -- 
> 1.8.0.2
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