blkcg: Introduce blkg_root_lookup()

This new function will be used in a later patch to verify whether a
queue has been dissociated from the cgroup controller before being
released.

Signed-off-by: Bart Van Assche <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Omar Sandoval <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Alexandru Moise <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

include/linux/blk-cgroup.h

@@ -341,6 +341,23 @@ static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg,
 	return __blkg_lookup(blkcg, q, false);
 }
 
+/**
+ * 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;
+}
+
 /**
  * blkg_to_pdata - get policy private data
  * @blkg: blkg of interest
@@ -864,6 +881,7 @@ static inline bool blk_cgroup_congested(void) { return false; }
 static inline void blkcg_schedule_throttle(struct request_queue *q, bool use_memdelay) { }
 
 static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, void *key) { return NULL; }
+static inline struct blkcg_gq *blkg_root_lookup(struct request_queue *q) { return NULL; }
 static inline int blkcg_init_queue(struct request_queue *q) { return 0; }
 static inline void blkcg_drain_queue(struct request_queue *q) { }
 static inline void blkcg_exit_queue(struct request_queue *q) { }