cgroup: remove cgroupfs_root->refcnt

Currently, cgroupfs_root and its ->top_cgroup are separated reference
counted and the latter's is ignored.  There's no reason to do this
separately.  This patch removes cgroupfs_root->refcnt and destroys
cgroupfs_root when the top_cgroup is released.

* cgroup_put() updated to ignore cgroup_is_dead() test for top
  cgroups.  cgroup_free_fn() updated to handle root destruction when
  releasing a top cgroup.

* As root destruction is now bounced through cgroup destruction, it is
  asynchronous.  Update cgroup_mount() so that it waits for pending
  release which is currently implemented using msleep().  Converting
  this to proper wait_queue isn't hard but likely unnecessary.

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

1 files changed, 38 insertions, 48 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index cffdb6e2ad08..03845c5d082b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -53,6 +53,7 @@
 #include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
 #include <linux/flex_array.h> /* used in cgroup_attach_task */
 #include <linux/kthread.h>
+#include <linux/delay.h>
 
 #include <linux/atomic.h>
 
@@ -728,37 +729,16 @@ static void cgroup_free_root(struct cgroupfs_root *root)
         }
 }
 
-static void cgroup_get_root(struct cgroupfs_root *root)
-{
-        /*
-         * The caller must ensure that @root is alive, which can be
-         * achieved by holding a ref on one of the member cgroups or
-         * following a registered reference to @root while holding
-         * cgroup_tree_mutex.
-         */
-        WARN_ON_ONCE(atomic_read(&root->refcnt) <= 0);
-        atomic_inc(&root->refcnt);
-}
-
-static void cgroup_put_root(struct cgroupfs_root *root)
+static void cgroup_destroy_root(struct cgroupfs_root *root)
 {
         struct cgroup *cgrp = &root->top_cgroup;
         struct cgrp_cset_link *link, *tmp_link;
         int ret;
 
-        /*
-         * @root's refcnt reaching zero and its deregistration should be
-         * atomic w.r.t. cgroup_tree_mutex.  This ensures that
-         * cgroup_get_root() is safe to invoke if @root is registered.
-         */
         mutex_lock(&cgroup_tree_mutex);
-        if (!atomic_dec_and_test(&root->refcnt)) {
-                mutex_unlock(&cgroup_tree_mutex);
-                return;
-        }
         mutex_lock(&cgroup_mutex);
 
-        BUG_ON(atomic_read(&root->nr_cgrps) != 1);
+        BUG_ON(atomic_read(&root->nr_cgrps));
         BUG_ON(!list_empty(&cgrp->children));
 
         /* Rebind all subsystems back to the default hierarchy */
@@ -929,21 +909,24 @@ static void cgroup_free_fn(struct work_struct *work)
         struct cgroup *cgrp = container_of(work, struct cgroup, destroy_work);
 
         atomic_dec(&cgrp->root->nr_cgrps);
-
-        /*
-         * We get a ref to the parent, and put the ref when this cgroup is
-         * being freed, so it's guaranteed that the parent won't be
-         * destroyed before its children.
-         */
-        cgroup_put(cgrp->parent);
-
-        /* put the root reference that we took when we created the cgroup */
-        cgroup_put_root(cgrp->root);
-
         cgroup_pidlist_destroy_all(cgrp);
 
-        kernfs_put(cgrp->kn);
-        kfree(cgrp);
+        if (cgrp->parent) {
+                /*
+                 * We get a ref to the parent, and put the ref when this
+                 * cgroup is being freed, so it's guaranteed that the
+                 * parent won't be destroyed before its children.
+                 */
+                cgroup_put(cgrp->parent);
+                kernfs_put(cgrp->kn);
+                kfree(cgrp);
+        } else {
+                /*
+                 * This is top cgroup's refcnt reaching zero, which
+                 * indicates that the root should be released.
+                 */
+                cgroup_destroy_root(cgrp->root);
+        }
 }
 
 static void cgroup_free_rcu(struct rcu_head *head)
@@ -965,7 +948,7 @@ static void cgroup_put(struct cgroup *cgrp)
 {
         if (!atomic_dec_and_test(&cgrp->refcnt))
                 return;
-        if (WARN_ON_ONCE(!cgroup_is_dead(cgrp)))
+        if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
                 return;
 
         /*
@@ -1356,7 +1339,6 @@ static void init_cgroup_root(struct cgroupfs_root *root)
 {
         struct cgroup *cgrp = &root->top_cgroup;
 
-        atomic_set(&root->refcnt, 1);
         INIT_LIST_HEAD(&root->root_list);
         atomic_set(&root->nr_cgrps, 1);
         cgrp->root = root;
@@ -1485,7 +1467,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
         struct cgroup_sb_opts opts;
         struct dentry *dentry;
         int ret;
-
+retry:
         mutex_lock(&cgroup_tree_mutex);
         mutex_lock(&cgroup_mutex);
 
@@ -1531,7 +1513,21 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                         }
                 }
 
-                cgroup_get_root(root);
+                /*
+                 * A root's lifetime is governed by its top cgroup.  Zero
+                 * ref indicate that the root is being destroyed.  Wait for
+                 * destruction to complete so that the subsystems are free.
+                 * We can use wait_queue for the wait but this path is
+                 * super cold.  Let's just sleep for a bit and retry.
+                 */
+                if (!atomic_inc_not_zero(&root->top_cgroup.refcnt)) {
+                        mutex_unlock(&cgroup_mutex);
+                        mutex_unlock(&cgroup_tree_mutex);
+                        msleep(10);
+                        goto retry;
+                }
+
+                ret = 0;
                 goto out_unlock;
         }
 
@@ -1558,7 +1554,7 @@ out_unlock:
 
         dentry = kernfs_mount(fs_type, flags, root->kf_root);
         if (IS_ERR(dentry))
-                cgroup_put_root(root);
+                cgroup_put(&root->top_cgroup);
         return dentry;
 }
 
@@ -1567,7 +1563,7 @@ static void cgroup_kill_sb(struct super_block *sb)
         struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
         struct cgroupfs_root *root = cgroup_root_from_kf(kf_root);
 
-        cgroup_put_root(root);
+        cgroup_put(&root->top_cgroup);
         kernfs_kill_sb(sb);
 }
 
@@ -3708,12 +3704,6 @@ static long cgroup_create(struct cgroup *parent, const char *name,
         /* allocation complete, commit to creation */
         list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
         atomic_inc(&root->nr_cgrps);
-
-        /*
-         * Grab a reference on the root and parent so that they don't get
-         * deleted while there are child cgroups.
-         */
-        cgroup_get_root(root);
         cgroup_get(parent);
 
         /*