linux-kernel - Re: [PATCH 02/13] blkcg: update blkg_lookup

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Date:   Tue, 27 Nov 2018 15:56:07 -0500
From:   Josef Bacik <josef@...icpanda.com>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Josef Bacik <josef@...icpanda.com>, kernel-team@...com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/13] blkcg: update blkg_lookup_create() to do locking

On Mon, Nov 26, 2018 at 04:19:35PM -0500, Dennis Zhou wrote:
> To know when to create a blkg, the general pattern is to do a
> blkg_lookup() and if that fails, lock and do the lookup again, and if
> that fails finally create. It doesn't make much sense for everyone who
> wants to do creation to write this themselves.
> 
> This changes blkg_lookup_create() to do locking and implement this
> pattern. The old blkg_lookup_create() is renamed to
> __blkg_lookup_create().  If a call site wants to do its own error
> handling or already owns the queue lock, they can use
> __blkg_lookup_create(). This will be used in upcoming patches.
> 
> Signed-off-by: Dennis Zhou <dennis@...nel.org>
> Reviewed-by: Josef Bacik <josef@...icpanda.com>
> Acked-by: Tejun Heo <tj@...nel.org>
> Reviewed-by: Liu Bo <bo.liu@...ux.alibaba.com>
> ---
>  block/blk-cgroup.c         | 30 +++++++++++++++++++++++++++---
>  block/blk-iolatency.c      |  2 +-
>  include/linux/blk-cgroup.h |  4 +++-
>  3 files changed, 31 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 63d226a084cd..b1ec98eeeae0 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -249,7 +249,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>  }
>  
>  /**
> - * blkg_lookup_create - lookup blkg, try to create one if not there
> + * __blkg_lookup_create - lookup blkg, try to create one if not there
>   * @blkcg: blkcg of interest
>   * @q: request_queue of interest
>   *
> @@ -262,8 +262,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>   * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
>   * dead and bypassing, returns ERR_PTR(-EBUSY).
>   */
> -struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
> -				    struct request_queue *q)
> +struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
> +				      struct request_queue *q)
>  {
>  	struct blkcg_gq *blkg;
>  
> @@ -293,6 +293,30 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
>  	}
>  }
>  
> +/**
> + * blkg_lookup_create - find or create a blkg
> + * @blkcg: target block cgroup
> + * @q: target request_queue
> + *
> + * This looks up or creates the blkg representing the unique pair
> + * of the blkcg and the request_queue.
> + */
> +struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
> +				    struct request_queue *q)
> +{
> +	struct blkcg_gq *blkg = blkg_lookup(blkcg, q);
> +
> +	if (unlikely(!blkg)) {
> +		spin_lock_irq(&q->queue_lock);
> +
> +		blkg = __blkg_lookup_create(blkcg, q);
> +
> +		spin_unlock_irq(&q->queue_lock);

No extra spaces here please, otherwise you can add

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef