cgroup: remove css_parent()

cgroup in general is moving towards using cgroup_subsys_state as the
fundamental structural component and css_parent() was introduced to
convert from using cgroup->parent to css->parent.  It was quite some
time ago and we're moving forward with making css more prominent.

This patch drops the trivial wrapper css_parent() and let the users
dereference css->parent.  While at it, explicitly mark fields of css
which are public and immutable.

v2: New usage from device_cgroup.c converted.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: "David S. Miller" <davem@davemloft.net>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Johannes Weiner <hannes@cmpxchg.org>

12 files changed, 34 insertions, 41 deletions

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 371fe8e92ab5..d692b29c083a 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -204,7 +204,7 @@ static inline struct blkcg *bio_blkcg(struct bio *bio)
  */
 static inline struct blkcg *blkcg_parent(struct blkcg *blkcg)
 {
-        return css_to_blkcg(css_parent(&blkcg->css));
+        return css_to_blkcg(blkcg->css.parent);
 }
 
 /**
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 1737db0c63fe..2549493d518d 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -48,22 +48,28 @@ enum cgroup_subsys_id {
 };
 #undef SUBSYS
 
-/* Per-subsystem/per-cgroup state maintained by the system. */
+/*
+ * Per-subsystem/per-cgroup state maintained by the system.  This is the
+ * fundamental structural building block that controllers deal with.
+ *
+ * Fields marked with "PI:" are public and immutable and may be accessed
+ * directly without synchronization.
+ */
 struct cgroup_subsys_state {
-        /* the cgroup that this css is attached to */
+        /* PI: the cgroup that this css is attached to */
         struct cgroup *cgroup;
 
-        /* the cgroup subsystem that this css is attached to */
+        /* PI: the cgroup subsystem that this css is attached to */
         struct cgroup_subsys *ss;
 
         /* reference count - access via css_[try]get() and css_put() */
         struct percpu_ref refcnt;
 
-        /* the parent css */
+        /* PI: the parent css */
         struct cgroup_subsys_state *parent;
 
         /*
-         * Subsys-unique ID.  0 is unused and root is always 1.  The
+         * PI: Subsys-unique ID.  0 is unused and root is always 1.  The
          * matching css can be looked up using css_from_id().
          */
         int id;
@@ -670,19 +676,6 @@ struct cgroup_subsys {
 #undef SUBSYS
 
 /**
- * css_parent - find the parent css
- * @css: the target cgroup_subsys_state
- *
- * Return the parent css of @css.  This function is guaranteed to return
- * non-NULL parent as long as @css isn't the root.
- */
-static inline
-struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
-{
-        return css->parent;
-}
-
-/**
  * task_css_set_check - obtain a task's css_set with extra access conditions
  * @task: the task to obtain css_set for
  * @__c: extra condition expression to be passed to rcu_dereference_check()
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0343d7ee6d62..929bbbc539e9 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3176,10 +3176,10 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
 
         /* no child, visit my or the closest ancestor's next sibling */
         while (pos != root) {
-                next = css_next_child(pos, css_parent(pos));
+                next = css_next_child(pos, pos->parent);
                 if (next)
                         return next;
-                pos = css_parent(pos);
+                pos = pos->parent;
         }
 
         return NULL;
@@ -3261,12 +3261,12 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
                 return NULL;
 
         /* if there's an unvisited sibling, visit its leftmost descendant */
-        next = css_next_child(pos, css_parent(pos));
+        next = css_next_child(pos, pos->parent);
         if (next)
                 return css_leftmost_descendant(next);
 
         /* no sibling left, visit parent */
-        return css_parent(pos);
+        return pos->parent;
 }
 
 static bool cgroup_has_live_children(struct cgroup *cgrp)
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 6b4e60e33a9a..a79e40f9d700 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -59,7 +59,7 @@ static inline struct freezer *task_freezer(struct task_struct *task)
 
 static struct freezer *parent_freezer(struct freezer *freezer)
 {
-        return css_freezer(css_parent(&freezer->css));
+        return css_freezer(freezer->css.parent);
 }
 
 bool cgroup_freezing(struct task_struct *task)
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 2f4b08b8db24..5b2a31082f4f 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -124,7 +124,7 @@ static inline struct cpuset *task_cs(struct task_struct *task)
 
 static inline struct cpuset *parent_cs(struct cpuset *cs)
 {
-        return css_cs(css_parent(&cs->css));
+        return css_cs(cs->css.parent);
 }
 
 #ifdef CONFIG_NUMA
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 268a45ea238c..ac61ad1a5f9f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7586,7 +7586,7 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
 {
         struct task_group *tg = css_tg(css);
-        struct task_group *parent = css_tg(css_parent(css));
+        struct task_group *parent = css_tg(css->parent);
 
         if (parent)
                 sched_online_group(tg, parent);
diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index c143ee380e3a..9cf350c94ec4 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -46,7 +46,7 @@ static inline struct cpuacct *task_ca(struct task_struct *tsk)
 
 static inline struct cpuacct *parent_ca(struct cpuacct *ca)
 {
-        return css_ca(css_parent(&ca->css));
+        return css_ca(ca->css.parent);
 }
 
 static DEFINE_PER_CPU(u64, root_cpuacct_cpuusage);
diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index a380681ab3cf..493f758445e7 100644
--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -52,7 +52,7 @@ static inline bool hugetlb_cgroup_is_root(struct hugetlb_cgroup *h_cg)
 static inline struct hugetlb_cgroup *
 parent_hugetlb_cgroup(struct hugetlb_cgroup *h_cg)
 {
-        return hugetlb_cgroup_from_css(css_parent(&h_cg->css));
+        return hugetlb_cgroup_from_css(h_cg->css.parent);
 }
 
 static inline bool hugetlb_cgroup_have_usage(struct hugetlb_cgroup *h_cg)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b638a79209ee..a5e0417b4f9a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1540,7 +1540,7 @@ static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
 int mem_cgroup_swappiness(struct mem_cgroup *memcg)
 {
         /* root ? */
-        if (!css_parent(&memcg->css))
+        if (!memcg->css.parent)
                 return vm_swappiness;
 
         return memcg->swappiness;
@@ -4909,7 +4909,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
 {
         int retval = 0;
         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-        struct mem_cgroup *parent_memcg = mem_cgroup_from_css(css_parent(&memcg->css));
+        struct mem_cgroup *parent_memcg = mem_cgroup_from_css(memcg->css.parent);
 
         mutex_lock(&memcg_create_mutex);
 
@@ -5207,8 +5207,8 @@ static void memcg_get_hierarchical_limit(struct mem_cgroup *memcg,
         if (!memcg->use_hierarchy)
                 goto out;
 
-        while (css_parent(&memcg->css)) {
-                memcg = mem_cgroup_from_css(css_parent(&memcg->css));
+        while (memcg->css.parent) {
+                memcg = mem_cgroup_from_css(memcg->css.parent);
                 if (!memcg->use_hierarchy)
                         break;
                 tmp = res_counter_read_u64(&memcg->res, RES_LIMIT);
@@ -5443,7 +5443,7 @@ static int mem_cgroup_swappiness_write(struct cgroup_subsys_state *css,
                                        struct cftype *cft, u64 val)
 {
         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-        struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
+        struct mem_cgroup *parent = mem_cgroup_from_css(memcg->css.parent);
 
         if (val > 100 || !parent)
                 return -EINVAL;
@@ -5790,7 +5790,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
         struct cftype *cft, u64 val)
 {
         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-        struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(&memcg->css));
+        struct mem_cgroup *parent = mem_cgroup_from_css(memcg->css.parent);
 
         /* cannot set to root cgroup and only 0 and 1 are allowed */
         if (!parent || !((val == 0) || (val == 1)))
@@ -6407,7 +6407,7 @@ static int
 mem_cgroup_css_online(struct cgroup_subsys_state *css)
 {
         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
-        struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(css));
+        struct mem_cgroup *parent = mem_cgroup_from_css(css->parent);
 
         if (css->id > MEM_CGROUP_ID_MAX)
                 return -ENOSPC;
diff --git a/net/core/netclassid_cgroup.c b/net/core/netclassid_cgroup.c
index 22931e1b99b4..30d903b19c62 100644
--- a/net/core/netclassid_cgroup.c
+++ b/net/core/netclassid_cgroup.c
@@ -42,7 +42,7 @@ cgrp_css_alloc(struct cgroup_subsys_state *parent_css)
 static int cgrp_css_online(struct cgroup_subsys_state *css)
 {
         struct cgroup_cls_state *cs = css_cls_state(css);
-        struct cgroup_cls_state *parent = css_cls_state(css_parent(css));
+        struct cgroup_cls_state *parent = css_cls_state(css->parent);
 
         if (parent)
                 cs->classid = parent->classid;
diff --git a/net/core/netprio_cgroup.c b/net/core/netprio_cgroup.c
index b990cefd906b..2f385b9bccc0 100644
--- a/net/core/netprio_cgroup.c
+++ b/net/core/netprio_cgroup.c
@@ -140,7 +140,7 @@ cgrp_css_alloc(struct cgroup_subsys_state *parent_css)
 
 static int cgrp_css_online(struct cgroup_subsys_state *css)
 {
-        struct cgroup_subsys_state *parent_css = css_parent(css);
+        struct cgroup_subsys_state *parent_css = css->parent;
         struct net_device *dev;
         int ret = 0;
 
diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 7dbac4061b1c..ce14a31b1337 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -182,7 +182,7 @@ static inline bool is_devcg_online(const struct dev_cgroup *devcg)
 static int devcgroup_online(struct cgroup_subsys_state *css)
 {
         struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
-        struct dev_cgroup *parent_dev_cgroup = css_to_devcgroup(css_parent(css));
+        struct dev_cgroup *parent_dev_cgroup = css_to_devcgroup(css->parent);
         int ret = 0;
 
         mutex_lock(&devcgroup_mutex);
@@ -455,7 +455,7 @@ static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
 static int parent_has_perm(struct dev_cgroup *childcg,
                                   struct dev_exception_item *ex)
 {
-        struct dev_cgroup *parent = css_to_devcgroup(css_parent(&childcg->css));
+        struct dev_cgroup *parent = css_to_devcgroup(childcg->css.parent);
 
         if (!parent)
                 return 1;
@@ -476,7 +476,7 @@ static int parent_has_perm(struct dev_cgroup *childcg,
 static bool parent_allows_removal(struct dev_cgroup *childcg,
                                   struct dev_exception_item *ex)
 {
-        struct dev_cgroup *parent = css_to_devcgroup(css_parent(&childcg->css));
+        struct dev_cgroup *parent = css_to_devcgroup(childcg->css.parent);
 
         if (!parent)
                 return true;
@@ -614,7 +614,7 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
         char temp[12];          /* 11 + 1 characters needed for a u32 */
         int count, rc = 0;
         struct dev_exception_item ex;
-        struct dev_cgroup *parent = css_to_devcgroup(css_parent(&devcgroup->css));
+        struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;