blkcg: let blkcg core manage per-queue blkg list and counter

With the previous patch to move blkg list heads and counters to
request_queue and blkg, logic to manage them in both policies are
almost identical and can be moved to blkcg core.

This patch moves blkg link logic into blkg_lookup_create(), implements
common blkg unlink code in blkg_destroy(), and updates
blkg_destory_all() so that it's policy specific and can skip root
group.  The updated blkg_destroy_all() is now used to both clear queue
for bypassing and elv switching, and release all blkgs on q exit.

This patch introduces a race window where policy [de]registration may
race against queue blkg clearing.  This can only be a problem on cfq
unload and shouldn't be a real problem in practice (and we have many
other places where this race already exists).  Future patches will
remove these unlikely races.

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, 97 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index c15d38307e1d..132941260e58 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -157,14 +157,6 @@ static void throtl_init_blkio_group(struct blkio_group *blkg)
         tg->iops[WRITE] = -1;
 }
 
-static void throtl_link_blkio_group(struct request_queue *q,
-                                    struct blkio_group *blkg)
-{
-        list_add(&blkg->q_node[BLKIO_POLICY_THROTL],
-                 &q->blkg_list[BLKIO_POLICY_THROTL]);
-        q->nr_blkgs[BLKIO_POLICY_THROTL]++;
-}
-
 static struct
 throtl_grp *throtl_lookup_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
 {
@@ -813,89 +805,6 @@ throtl_schedule_delayed_work(struct throtl_data *td, unsigned long delay)
         }
 }
 
-static void
-throtl_destroy_tg(struct throtl_data *td, struct throtl_grp *tg)
-{
-        struct blkio_group *blkg = tg_to_blkg(tg);
-
-        /* Something wrong if we are trying to remove same group twice */
-        WARN_ON_ONCE(list_empty(&blkg->q_node[BLKIO_POLICY_THROTL]));
-
-        list_del_init(&blkg->q_node[BLKIO_POLICY_THROTL]);
-
-        /*
-         * Put the reference taken at the time of creation so that when all
-         * queues are gone, group can be destroyed.
-         */
-        blkg_put(tg_to_blkg(tg));
-        td->queue->nr_blkgs[BLKIO_POLICY_THROTL]--;
-}
-
-static bool throtl_release_tgs(struct throtl_data *td, bool release_root)
-{
-        struct request_queue *q = td->queue;
-        struct blkio_group *blkg, *n;
-        bool empty = true;
-
-        list_for_each_entry_safe(blkg, n, &q->blkg_list[BLKIO_POLICY_THROTL],
-                                 q_node[BLKIO_POLICY_THROTL]) {
-                struct throtl_grp *tg = blkg_to_tg(blkg);
-
-                /* skip root? */
-                if (!release_root && tg == td->root_tg)
-                        continue;
-
-                /*
-                 * If cgroup removal path got to blk_group first and removed
-                 * it from cgroup list, then it will take care of destroying
-                 * cfqg also.
-                 */
-                if (!blkiocg_del_blkio_group(blkg))
-                        throtl_destroy_tg(td, tg);
-                else
-                        empty = false;
-        }
-        return empty;
-}
-
-/*
- * Blk cgroup controller notification saying that blkio_group object is being
- * delinked as associated cgroup object is going away. That also means that
- * no new IO will come in this group. So get rid of this group as soon as
- * any pending IO in the group is finished.
- *
- * This function is called under rcu_read_lock(). @q is the rcu protected
- * pointer. That means @q is a valid request_queue pointer as long as we
- * are rcu read lock.
- *
- * @q was fetched from blkio_group under blkio_cgroup->lock. That means
- * it should not be NULL as even if queue was going away, cgroup deltion
- * path got to it first.
- */
-void throtl_unlink_blkio_group(struct request_queue *q,
-                               struct blkio_group *blkg)
-{
-        unsigned long flags;
-
-        spin_lock_irqsave(q->queue_lock, flags);
-        throtl_destroy_tg(q->td, blkg_to_tg(blkg));
-        spin_unlock_irqrestore(q->queue_lock, flags);
-}
-
-static bool throtl_clear_queue(struct request_queue *q)
-{
-        lockdep_assert_held(q->queue_lock);
-
-        /*
-         * Clear tgs but leave the root one alone.  This is necessary
-         * because root_tg is expected to be persistent and safe because
-         * blk-throtl can never be disabled while @q is alive.  This is a
-         * kludge to prepare for unified blkg.  This whole function will be
-         * removed soon.
-         */
-        return throtl_release_tgs(q->td, false);
-}
-
 static void throtl_update_blkio_group_common(struct throtl_data *td,
                                 struct throtl_grp *tg)
 {
@@ -960,9 +869,6 @@ static void throtl_shutdown_wq(struct request_queue *q)
 static struct blkio_policy_type blkio_policy_throtl = {
         .ops = {
                 .blkio_init_group_fn = throtl_init_blkio_group,
-                .blkio_link_group_fn = throtl_link_blkio_group,
-                .blkio_unlink_group_fn = throtl_unlink_blkio_group,
-                .blkio_clear_queue_fn = throtl_clear_queue,
                 .blkio_update_group_read_bps_fn =
                                         throtl_update_blkio_group_read_bps,
                 .blkio_update_group_write_bps_fn =
@@ -1148,12 +1054,11 @@ void blk_throtl_exit(struct request_queue *q)
 
         throtl_shutdown_wq(q);
 
-        spin_lock_irq(q->queue_lock);
-        throtl_release_tgs(td, true);
+        blkg_destroy_all(q, BLKIO_POLICY_THROTL, true);
 
         /* If there are other groups */
+        spin_lock_irq(q->queue_lock);
         wait = q->nr_blkgs[BLKIO_POLICY_THROTL];
-
         spin_unlock_irq(q->queue_lock);
 
         /*