blkcg: shoot down blkio_groups on elevator switch

Elevator switch may involve changes to blkcg policies.  Implement
shoot down of blkio_groups.

Combined with the previous bypass updates, the end goal is updating
blkcg core such that it can ensure that blkcg's being affected become
quiescent and don't have any per-blkg data hanging around before
commencing any policy updates.  Until queues are made aware of the
policies that applies to them, as an interim step, all per-policy blkg
data will be shot down.

* blk-throtl doesn't need this change as it can't be disabled for a
  live queue; however, update it anyway as the scheduled blkg
  unification requires this behavior change.  This means that
  blk-throtl configuration will be unnecessarily lost over elevator
  switch.  This oddity will be removed after blkcg learns to associate
  individual policies with request_queues.

* blk-throtl dosen't shoot down root_tg.  This is to ease transition.
  Unified blkg will always have persistent root group and not shooting
  down root_tg for now eases transition to that point by avoiding
  having to update td->root_tg and is safe as blk-throtl can never be
  disabled

-v2: Vivek pointed out that group list is not guaranteed to be empty
     on return from clear function if it raced cgroup removal and
     lost.  Fix it by waiting a bit and retrying.  This kludge will
     soon be removed once locking is updated such that blkg is never
     in limbo state between blkcg and request_queue locks.

     blk-throtl no longer shoots down root_tg to avoid breaking
     td->root_tg.

     Also, Nest queue_lock inside blkio_list_lock not the other way
     around to avoid introduce possible deadlock via blkcg lock.

-v3: blkcg_clear_queue() repositioned and renamed to
     blkg_destroy_all() to increase consistency with later changes.
     cfq_clear_queue() updated to check q->elevator before
     dereferencing it to avoid NULL dereference on not fully
     initialized queues (used by later change).

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, 19 insertions, 1 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 72680a6715f..61693d3404d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1225,10 +1225,11 @@ static void cfq_destroy_cfqg(struct cfq_data *cfqd, struct cfq_group *cfqg)
         cfq_put_cfqg(cfqg);
 }
 
-static void cfq_release_cfq_groups(struct cfq_data *cfqd)
+static bool cfq_release_cfq_groups(struct cfq_data *cfqd)
 {
         struct hlist_node *pos, *n;
         struct cfq_group *cfqg;
+        bool empty = true;
 
         hlist_for_each_entry_safe(cfqg, pos, n, &cfqd->cfqg_list, cfqd_node) {
                 /*
@@ -1238,7 +1239,10 @@ static void cfq_release_cfq_groups(struct cfq_data *cfqd)
                  */
                 if (!cfq_blkiocg_del_blkio_group(&cfqg->blkg))
                         cfq_destroy_cfqg(cfqd, cfqg);
+                else
+                        empty = false;
         }
+        return empty;
 }
 
 /*
@@ -1265,6 +1269,19 @@ static void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
         spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
 }
 
+static struct elevator_type iosched_cfq;
+
+static bool cfq_clear_queue(struct request_queue *q)
+{
+        lockdep_assert_held(q->queue_lock);
+
+        /* shoot down blkgs iff the current elevator is cfq */
+        if (!q->elevator || q->elevator->type != &iosched_cfq)
+                return true;
+
+        return cfq_release_cfq_groups(q->elevator->elevator_data);
+}
+
 #else /* GROUP_IOSCHED */
 static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
 {
@@ -3875,6 +3892,7 @@ static struct elevator_type iosched_cfq = {
 static struct blkio_policy_type blkio_policy_cfq = {
         .ops = {
                 .blkio_unlink_group_fn =        cfq_unlink_blkio_group,
+                .blkio_clear_queue_fn = cfq_clear_queue,
                 .blkio_update_group_weight_fn = cfq_update_blkio_group_weight,
         },
         .plid = BLKIO_POLICY_PROP,