cgroup: introduce CSS_ONLINE flag and on/offline_css() helpers

New helpers on/offline_css() respectively wrap ->post_create() and
->pre_destroy() invocations.  online_css() sets CSS_ONLINE after
->post_create() is complete and offline_css() invokes ->pre_destroy()
iff CSS_ONLINE is set and clears it while also handling the temporary
dropping of cgroup_mutex.

This patch doesn't introduce any behavior change at the moment but
will be used to improve cgroup_create() failure path and allow
->post_create() to fail.

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

1 files changed, 42 insertions, 23 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 4412d9694f13..78a3d5c0968e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4035,6 +4035,42 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
         INIT_WORK(&css->dput_work, css_dput_fn);
 }
 
+/* invoke ->post_create() on a new CSS and mark it online */
+static void online_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
+{
+        lockdep_assert_held(&cgroup_mutex);
+
+        if (ss->post_create)
+                ss->post_create(cgrp);
+        cgrp->subsys[ss->subsys_id]->flags |= CSS_ONLINE;
+}
+
+/* if the CSS is online, invoke ->pre_destory() on it and mark it offline */
+static void offline_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
+        __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
+{
+        struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
+
+        lockdep_assert_held(&cgroup_mutex);
+
+        if (!(css->flags & CSS_ONLINE))
+                return;
+
+        /*
+         * pre_destroy() should be called with cgroup_mutex unlocked.  See
+         * 3fa59dfbc3 ("cgroup: fix potential deadlock in pre_destroy") for
+         * details.  This temporary unlocking should go away once
+         * cgroup_mutex is unexported from controllers.
+         */
+        if (ss->pre_destroy) {
+                mutex_unlock(&cgroup_mutex);
+                ss->pre_destroy(cgrp);
+                mutex_lock(&cgroup_mutex);
+        }
+
+        cgrp->subsys[ss->subsys_id]->flags &= ~CSS_ONLINE;
+}
+
 /*
  * cgroup_create - create a cgroup
  * @parent: cgroup that will be parent of the new cgroup
@@ -4137,8 +4173,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 dget(dentry);
 
                 /* creation succeeded, notify subsystems */
-                if (ss->post_create)
-                        ss->post_create(cgrp);
+                online_css(ss, cgrp);
         }
 
         err = cgroup_populate_dir(cgrp, true, root->subsys_mask);
@@ -4240,18 +4275,9 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         }
         set_bit(CGRP_REMOVED, &cgrp->flags);
 
-        /*
-         * Tell subsystems to initate destruction.  pre_destroy() should be
-         * called with cgroup_mutex unlocked.  See 3fa59dfbc3 ("cgroup: fix
-         * potential deadlock in pre_destroy") for details.  This temporary
-         * unlocking should go away once cgroup_mutex is unexported from
-         * controllers.
-         */
-        mutex_unlock(&cgroup_mutex);
+        /* tell subsystems to initate destruction */
         for_each_subsys(cgrp->root, ss)
-                if (ss->pre_destroy)
-                        ss->pre_destroy(cgrp);
-        mutex_lock(&cgroup_mutex);
+                offline_css(ss, cgrp);
 
         /*
          * Put all the base refs.  Each css holds an extra reference to the
@@ -4354,9 +4380,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         BUG_ON(!list_empty(&init_task.tasks));
 
         ss->active = 1;
-
-        if (ss->post_create)
-                ss->post_create(dummytop);
+        online_css(ss, dummytop);
 
         mutex_unlock(&cgroup_mutex);
 
@@ -4469,9 +4493,7 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
         write_unlock(&css_set_lock);
 
         ss->active = 1;
-
-        if (ss->post_create)
-                ss->post_create(dummytop);
+        online_css(ss, dummytop);
 
         /* success! */
         mutex_unlock(&cgroup_mutex);
@@ -4501,12 +4523,9 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
          */
         BUG_ON(ss->root != &rootnode);
 
-        /* ->pre_destroy() should be called outside cgroup_mutex for now */
-        if (ss->pre_destroy)
-                ss->pre_destroy(dummytop);
-
         mutex_lock(&cgroup_mutex);
 
+        offline_css(ss, dummytop);
         ss->active = 0;
 
         if (ss->use_id) {