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>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index a0fc641..f4a9c98 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -82,6 +82,7 @@ struct cgroup_subsys_state {
 /* bits in struct cgroup_subsys_state flags field */
 enum {
        CSS_ROOT        = (1 << 0), /* this CSS is the root of the subsystem */
+       CSS_ONLINE      = (1 << 1), /* between ->post_create() and ->pre_destroy() */
 };
 
 /* Caller must verify that the css is not for root cgroup */

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 4412d96..78a3d5c 100644 (file)
--- 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) {