percpu-refcount: require percpu_ref to be exited explicitly

Currently, a percpu_ref undoes percpu_ref_init() automatically by
freeing the allocated percpu area when the percpu_ref is killed.
While seemingly convenient, this has the following niggles.

* It's impossible to re-init a released reference counter without
  going through re-allocation.

* In the similar vein, it's impossible to initialize a percpu_ref
  count with static percpu variables.

* We need and have an explicit destructor anyway for failure paths -
  percpu_ref_cancel_init().

This patch removes the automatic percpu counter freeing in
percpu_ref_kill_rcu() and repurposes percpu_ref_cancel_init() into a
generic destructor now named percpu_ref_exit().  percpu_ref_destroy()
is considered but it gets confusing with percpu_ref_kill() while
"exit" clearly indicates that it's the counterpart of
percpu_ref_init().

All percpu_ref_cancel_init() users are updated to invoke
percpu_ref_exit() instead and explicit percpu_ref_exit() calls are
added to the destruction path of all percpu_ref users.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Cc: Li Zefan <lizefan@huawei.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7868fc3c0bc5..c06aa5e257a8 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1638,7 +1638,7 @@ destroy_root:
 exit_root_id:
         cgroup_exit_root_id(root);
 cancel_ref:
-        percpu_ref_cancel_init(&root_cgrp->self.refcnt);
+        percpu_ref_exit(&root_cgrp->self.refcnt);
 out:
         free_cgrp_cset_links(&tmp_links);
         return ret;
@@ -4133,6 +4133,8 @@ static void css_free_work_fn(struct work_struct *work)
                 container_of(work, struct cgroup_subsys_state, destroy_work);
         struct cgroup *cgrp = css->cgroup;
 
+        percpu_ref_exit(&css->refcnt);
+
         if (css->ss) {
                 /* css free path */
                 if (css->parent)
@@ -4330,7 +4332,7 @@ err_list_del:
 err_free_id:
         cgroup_idr_remove(&ss->css_idr, css->id);
 err_free_percpu_ref:
-        percpu_ref_cancel_init(&css->refcnt);
+        percpu_ref_exit(&css->refcnt);
 err_free_css:
         call_rcu(&css->rcu_head, css_free_rcu_fn);
         return err;
@@ -4441,7 +4443,7 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 out_free_id:
         cgroup_idr_remove(&root->cgroup_idr, cgrp->id);
 out_cancel_ref:
-        percpu_ref_cancel_init(&cgrp->self.refcnt);
+        percpu_ref_exit(&cgrp->self.refcnt);
 out_free_cgrp:
         kfree(cgrp);
 out_unlock: