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>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c86a93abe83dd6cf7dc572452dc98daa90e7cc40..81bfd0268e93c485c6b645f2ec193ca19d156d46 100644 (file)
--- 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 1a68241ca835bc601954134c583e3b27337d214d..49bfd7b0bbdaa05130235d57537f7f3eeddbd8b9 100644 (file)
--- 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);
@@ -5507,10 +5500,6 @@ 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,