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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d12c29f42febbfcd06fb497ff0c5c5fa0747d3a3..329fde82ef7cd1b5ca2e8cc8a23470325e977151 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -242,6 +242,21 @@ static int notify_on_release(const struct cgroup *cgrp)
        return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
 }
 
+/**
+ * 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
@@ -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