blkcg: kill the mind-bending blkg->dev

blkg->dev is dev_t recording the device number of the block device for
the associated request_queue.  It is used to identify the associated
block device when printing out configuration or stats.

This is redundant to begin with.  A blkg is an association between a
cgroup and a request_queue and it of course is possible to reach
request_queue from blkg and synchronization conventions are in place
for safe q dereferencing, so this shouldn't be necessary from the
beginning.  Furthermore, it's initialized by sscanf()ing the device
name of backing_dev_info.  The mind boggles.

Anyways, if blkg is visible under rcu lock, we *know* that the
associated request_queue hasn't gone away yet and its bdi is
registered and alive - blkg can't be created for request_queue which
hasn't been fully initialized and it can't go away before blkg is
removed.

Let stat and conf read functions get device name from
blkg->q->backing_dev_info.dev and pass it down to printing functions
and remove blkg->dev.

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, 0 insertions, 21 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 08d4fdd188fa..f67d109eb974 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1052,20 +1052,7 @@ static void cfq_link_blkio_group(struct request_queue *q,
                                  struct blkio_group *blkg)
 {
         struct cfq_data *cfqd = q->elevator->elevator_data;
-        struct backing_dev_info *bdi = &q->backing_dev_info;
         struct cfq_group *cfqg = cfqg_of_blkg(blkg);
-        unsigned int major, minor;
-
-        /*
-         * Add group onto cgroup list. It might happen that bdi->dev is
-         * not initialized yet. Initialize this new group without major
-         * and minor info and this info will be filled in once a new thread
-         * comes for IO.
-         */
-        if (bdi->dev) {
-                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                blkg->dev = MKDEV(major, minor);
-        }
 
         cfqd->nr_blkcg_linked_grps++;
 
@@ -1104,7 +1091,6 @@ static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
                                                 struct blkio_cgroup *blkcg)
 {
         struct request_queue *q = cfqd->queue;
-        struct backing_dev_info *bdi = &q->backing_dev_info;
         struct cfq_group *cfqg = NULL;
 
         /* avoid lookup for the common case where there's no blkio cgroup */
@@ -1118,13 +1104,6 @@ static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
                         cfqg = cfqg_of_blkg(blkg);
         }
 
-        if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
-                unsigned int major, minor;
-
-                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                cfqg->blkg.dev = MKDEV(major, minor);
-        }
-
         return cfqg;
 }