cgroup: remove cgroup->count and use

cgroup->count tracks the number of css_sets associated with the cgroup
and used only to verify that no css_set is associated when the cgroup
is being destroyed.  It's superflous as the destruction path can
simply check whether cgroup->cset_links is empty instead.

Drop cgroup->count and check ->cset_links directly from
cgroup_destroy_locked().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

2 files changed, 5 insertions, 22 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c86a93abe83d..81bfd0268e93 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -169,12 +169,6 @@ struct cgroup_name {
 struct cgroup {
         unsigned long flags;            /* "unsigned long" so bitops work */
 
-        /*
-         * count users of this cgroup. >0 means busy, but doesn't
-         * necessarily indicate the number of tasks in the cgroup
-         */
-        atomic_t count;
-
         int id;                         /* ida allocated in-hierarchy ID */
 
         /*
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1a68241ca835..49bfd7b0bbda 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -408,8 +408,7 @@ static void __put_css_set(struct css_set *cset, int taskexit)
                 list_del(&link->cgrp_link);
 
                 /* @cgrp can't go away while we're holding css_set_lock */
-                if (atomic_dec_and_test(&cgrp->count) &&
-                    notify_on_release(cgrp)) {
+                if (list_empty(&cgrp->cset_links) && notify_on_release(cgrp)) {
                         if (taskexit)
                                 set_bit(CGRP_RELEASABLE, &cgrp->flags);
                         check_for_release(cgrp);
@@ -616,7 +615,6 @@ static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
         link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
         link->cset = cset;
         link->cgrp = cgrp;
-        atomic_inc(&cgrp->count);
         list_move(&link->cset_link, &cgrp->cset_links);
         /*
          * Always add links to the tail of the list so that the list
@@ -4370,11 +4368,11 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         lockdep_assert_held(&cgroup_mutex);
 
         /*
-         * css_set_lock prevents @cgrp from being removed while
-         * __put_css_set() is in progress.
+         * css_set_lock synchronizes access to ->cset_links and prevents
+         * @cgrp from being removed while __put_css_set() is in progress.
          */
         read_lock(&css_set_lock);
-        empty = !atomic_read(&cgrp->count) && list_empty(&cgrp->children);
+        empty = list_empty(&cgrp->cset_links) && list_empty(&cgrp->children);
         read_unlock(&css_set_lock);
         if (!empty)
                 return -EBUSY;
@@ -5054,7 +5052,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 static void check_for_release(struct cgroup *cgrp)
 {
         if (cgroup_is_releasable(cgrp) &&
-            !atomic_read(&cgrp->count) && list_empty(&cgrp->children)) {
+            list_empty(&cgrp->cset_links) && list_empty(&cgrp->children)) {
                 /*
                  * Control Group is currently removeable. If it's not
                  * already queued for a userspace notification, queue
@@ -5422,11 +5420,6 @@ static void debug_css_free(struct cgroup *cont)
         kfree(cont->subsys[debug_subsys_id]);
 }
 
-static u64 cgroup_refcount_read(struct cgroup *cont, struct cftype *cft)
-{
-        return atomic_read(&cont->count);
-}
-
 static u64 debug_taskcount_read(struct cgroup *cont, struct cftype *cft)
 {
         return cgroup_task_count(cont);
@@ -5508,10 +5501,6 @@ static u64 releasable_read(struct cgroup *cgrp, struct cftype *cft)
 
 static struct cftype debug_files[] =  {
         {
-                .name = "cgroup_refcount",
-                .read_u64 = cgroup_refcount_read,
-        },
-        {
                 .name = "taskcount",
                 .read_u64 = debug_taskcount_read,
         },