Re: [PATCH v5 1/3] blkcg: Introduce blkg_root_lookup()

On Thu, 2018-08-09 at 12:56 -0700, Tejun Heo wrote:

Hello,

On Thu, Aug 09, 2018 at 07:53:36AM -0700, Bart Van Assche wrote:

...

+/**

• • blkg_lookup - look up blkg for the specified request queue
• • @q: request_queue of interest
• • Lookup blkg for @q at the root level. See also blkg_lookup().
• */

+static inline struct blkcg_gq *blkg_root_lookup(struct request_queue *q) +{

• struct blkcg_gq *blkg;
• rcu_read_lock();
• blkg = blkg_lookup(&blkcg_root, q);
• rcu_read_unlock();
• return blkg;

+}

So, root blkg is always available as long as the request_queue is alive. Sth like the following would be simpler?

static inline struct blkcg_gq *blk_queue_root_blkg(struct request_queue *q) { return q->root_blkg; }

That would not only be simpler, it would also avoid that blk_queue_root_blkg() returns NULL if bypass mode is enabled and the request queue is still associated with the block cgroup controller. How do you want to proceed with this change?

Bart.