blkcg: let blkio_group point to blkio_cgroup directly

Currently, blkg points to the associated blkcg via its css_id.  This
unnecessarily complicates dereferencing blkcg.  Let blkg hold a
reference to the associated blkcg and point directly to it and disable
css_id on blkio_subsys.

This change requires splitting blkiocg_destroy() into
blkiocg_pre_destroy() and blkiocg_destroy() so that all blkg's can be
destroyed and all the blkcg references held by them dropped during
cgroup removal.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 0 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 52a429397d3b..fe6a442b8482 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -169,6 +169,9 @@ static void throtl_put_tg(struct throtl_grp *tg)
         if (!atomic_dec_and_test(&tg->ref))
                 return;
 
+        /* release the extra blkcg reference this blkg has been holding */
+        css_put(&tg->blkg.blkcg->css);
+
         /*
          * A group is freed in rcu manner. But having an rcu lock does not
          * mean that one can access all the fields of blkg and assume these