cgroup: implement for_each_css()

There are enough places where css's of a cgroup are iterated, which
currently uses for_each_root_subsys() + explicit cgroup_css().  This
patch implements for_each_css() and replaces the above combination
with it.

This patch doesn't introduce any behavior changes.

v2: Updated to apply cleanly on top of v2 of "cgroup: fix css leaks on
    online_css() failure"

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

1 files changed, 31 insertions, 26 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d12c29f42feb..329fde82ef7c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -243,6 +243,21 @@ static int notify_on_release(const struct cgroup *cgrp)
 }
 
 /**
+ * for_each_css - iterate all css's of a cgroup
+ * @css: the iteration cursor
+ * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
+ * @cgrp: the target cgroup to iterate css's of
+ *
+ * Should be called under cgroup_mutex.
+ */
+#define for_each_css(css, ssid, cgrp)                                   \
+        for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)        \
+                if (!((css) = rcu_dereference_check(                    \
+                                (cgrp)->subsys[(ssid)],                 \
+                                lockdep_is_held(&cgroup_mutex)))) { }   \
+                else
+
+/**
  * for_each_subsys - iterate all loaded cgroup subsystems
  * @ss: the iteration cursor
  * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
@@ -1942,8 +1957,8 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
                               bool threadgroup)
 {
         int retval, i, group_size;
-        struct cgroup_subsys *ss, *failed_ss = NULL;
         struct cgroupfs_root *root = cgrp->root;
+        struct cgroup_subsys_state *css, *failed_css = NULL;
         /* threadgroup list cursor and array */
         struct task_struct *leader = tsk;
         struct task_and_cgroup *tc;
@@ -2016,13 +2031,11 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
         /*
          * step 1: check that we can legitimately attach to the cgroup.
          */
-        for_each_root_subsys(root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
-
-                if (ss->can_attach) {
-                        retval = ss->can_attach(css, &tset);
+        for_each_css(css, i, cgrp) {
+                if (css->ss->can_attach) {
+                        retval = css->ss->can_attach(css, &tset);
                         if (retval) {
-                                failed_ss = ss;
+                                failed_css = css;
                                 goto out_cancel_attach;
                         }
                 }
@@ -2058,12 +2071,9 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
         /*
          * step 4: do subsystem attach callbacks.
          */
-        for_each_root_subsys(root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
-
-                if (ss->attach)
-                        ss->attach(css, &tset);
-        }
+        for_each_css(css, i, cgrp)
+                if (css->ss->attach)
+                        css->ss->attach(css, &tset);
 
         /*
          * step 5: success! and cleanup
@@ -2080,13 +2090,11 @@ out_put_css_set_refs:
         }
 out_cancel_attach:
         if (retval) {
-                for_each_root_subsys(root, ss) {
-                        struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
-
-                        if (ss == failed_ss)
+                for_each_css(css, i, cgrp) {
+                        if (css == failed_css)
                                 break;
-                        if (ss->cancel_attach)
-                                ss->cancel_attach(css, &tset);
+                        if (css->ss->cancel_attach)
+                                css->ss->cancel_attach(css, &tset);
                 }
         }
 out_free_group_list:
@@ -4375,9 +4383,10 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
         struct dentry *d = cgrp->dentry;
-        struct cgroup_subsys *ss;
+        struct cgroup_subsys_state *css;
         struct cgroup *child;
         bool empty;
+        int ssid;
 
         lockdep_assert_held(&d->d_inode->i_mutex);
         lockdep_assert_held(&cgroup_mutex);
@@ -4413,12 +4422,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
          * will be invoked to perform the rest of destruction once the
          * percpu refs of all css's are confirmed to be killed.
          */
-        for_each_root_subsys(cgrp->root, ss) {
-                struct cgroup_subsys_state *css = cgroup_css(cgrp, ss);
-
-                if (css)
-                        kill_css(css);
-        }
+        for_each_css(css, ssid, cgrp)
+                kill_css(css);
 
         /*
          * Mark @cgrp dead.  This prevents further task migration and child