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, 2 insertions, 49 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 791b10719e4..52a429397d3 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -212,50 +212,12 @@ static struct blkio_group *throtl_alloc_blkio_group(struct request_queue *q,
         return &tg->blkg;
 }
 
-static void
-__throtl_tg_fill_dev_details(struct throtl_data *td, struct throtl_grp *tg)
-{
-        struct backing_dev_info *bdi = &td->queue->backing_dev_info;
-        unsigned int major, minor;
-
-        if (!tg || tg->blkg.dev)
-                return;
-
-        /*
-         * Fill in device details for a group which might not have been
-         * filled at group creation time as queue was being instantiated
-         * and driver had not attached a device yet
-         */
-        if (bdi->dev && dev_name(bdi->dev)) {
-                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                tg->blkg.dev = MKDEV(major, minor);
-        }
-}
-
-/*
- * Should be called with without queue lock held. Here queue lock will be
- * taken rarely. It will be taken only once during life time of a group
- * if need be
- */
-static void
-throtl_tg_fill_dev_details(struct throtl_data *td, struct throtl_grp *tg)
-{
-        if (!tg || tg->blkg.dev)
-                return;
-
-        spin_lock_irq(td->queue->queue_lock);
-        __throtl_tg_fill_dev_details(td, tg);
-        spin_unlock_irq(td->queue->queue_lock);
-}
-
 static void throtl_link_blkio_group(struct request_queue *q,
                                     struct blkio_group *blkg)
 {
         struct throtl_data *td = q->td;
         struct throtl_grp *tg = tg_of_blkg(blkg);
 
-        __throtl_tg_fill_dev_details(td, tg);
-
         hlist_add_head(&tg->tg_node, &td->tg_list);
         td->nr_undestroyed_grps++;
 }
@@ -263,20 +225,14 @@ static void throtl_link_blkio_group(struct request_queue *q,
 static struct
 throtl_grp *throtl_lookup_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
 {
-        struct throtl_grp *tg = NULL;
-
         /*
          * This is the common case when there are no blkio cgroups.
          * Avoid lookup in this case
          */
         if (blkcg == &blkio_root_cgroup)
-                tg = td->root_tg;
-        else
-                tg = tg_of_blkg(blkg_lookup(blkcg, td->queue,
-                                            BLKIO_POLICY_THROTL));
+                return td->root_tg;
 
-        __throtl_tg_fill_dev_details(td, tg);
-        return tg;
+        return tg_of_blkg(blkg_lookup(blkcg, td->queue, BLKIO_POLICY_THROTL));
 }
 
 static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
@@ -303,7 +259,6 @@ static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
                         tg = td->root_tg;
         }
 
-        __throtl_tg_fill_dev_details(td, tg);
         return tg;
 }
 
@@ -1090,8 +1045,6 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
         blkcg = task_blkio_cgroup(current);
         tg = throtl_lookup_tg(td, blkcg);
         if (tg) {
-                throtl_tg_fill_dev_details(td, tg);
-
                 if (tg_no_rule_group(tg, rw)) {
                         blkiocg_update_dispatch_stats(&tg->blkg, bio->bi_size,
                                         rw, rw_is_sync(bio->bi_rw));