cgroup: factor out cgroup_kn_lock_live() and cgroup_kn_unlock()

cgroup_mkdir(), cgroup_rmdir() and cgroup_subtree_control_write()
share the logic to break active protection so that they can grab
cgroup_tree_mutex which nests above active protection and/or remove
self.  Factor out this logic into cgroup_kn_lock_live() and
cgroup_kn_unlock().

This patch doesn't introduce any functional changes.

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

1 files changed, 90 insertions, 67 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b49e63d5386b..21739e481006 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1093,6 +1093,75 @@ static void cgroup_put(struct cgroup *cgrp)
         call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
 }
 
+/**
+ * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
+ * @kn: the kernfs_node being serviced
+ *
+ * This helper undoes cgroup_kn_lock_live() and should be invoked before
+ * the method finishes if locking succeeded.  Note that once this function
+ * returns the cgroup returned by cgroup_kn_lock_live() may become
+ * inaccessible any time.  If the caller intends to continue to access the
+ * cgroup, it should pin it before invoking this function.
+ */
+static void cgroup_kn_unlock(struct kernfs_node *kn)
+{
+        struct cgroup *cgrp;
+
+        if (kernfs_type(kn) == KERNFS_DIR)
+                cgrp = kn->priv;
+        else
+                cgrp = kn->parent->priv;
+
+        mutex_unlock(&cgroup_mutex);
+        mutex_unlock(&cgroup_tree_mutex);
+
+        kernfs_unbreak_active_protection(kn);
+        cgroup_put(cgrp);
+}
+
+/**
+ * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
+ * @kn: the kernfs_node being serviced
+ *
+ * This helper is to be used by a cgroup kernfs method currently servicing
+ * @kn.  It breaks the active protection, performs cgroup locking and
+ * verifies that the associated cgroup is alive.  Returns the cgroup if
+ * alive; otherwise, %NULL.  A successful return should be undone by a
+ * matching cgroup_kn_unlock() invocation.
+ *
+ * Any cgroup kernfs method implementation which requires locking the
+ * associated cgroup should use this helper.  It avoids nesting cgroup
+ * locking under kernfs active protection and allows all kernfs operations
+ * including self-removal.
+ */
+static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
+{
+        struct cgroup *cgrp;
+
+        if (kernfs_type(kn) == KERNFS_DIR)
+                cgrp = kn->priv;
+        else
+                cgrp = kn->parent->priv;
+
+        /*
+         * We're gonna grab cgroup_tree_mutex which nests outside kernfs
+         * active_ref.  cgroup liveliness check alone provides enough
+         * protection against removal.  Ensure @cgrp stays accessible and
+         * break the active_ref protection.
+         */
+        cgroup_get(cgrp);
+        kernfs_break_active_protection(kn);
+
+        mutex_lock(&cgroup_tree_mutex);
+        mutex_lock(&cgroup_mutex);
+
+        if (!cgroup_is_dead(cgrp))
+                return cgrp;
+
+        cgroup_kn_unlock(kn);
+        return NULL;
+}
+
 static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 {
         char name[CGROUP_FILE_NAME_MAX];
@@ -2541,7 +2610,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                                             loff_t off)
 {
         unsigned int enable = 0, disable = 0;
-        struct cgroup *cgrp = of_css(of)->cgroup, *child;
+        struct cgroup *cgrp, *child;
         struct cgroup_subsys *ss;
         char *tok;
         int ssid, ret;
@@ -2573,20 +2642,9 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                         return -EINVAL;
         }
 
-        /*
-         * We're gonna grab cgroup_tree_mutex which nests outside kernfs
-         * active_ref.  cgroup_lock_live_group() already provides enough
-         * protection.  Ensure @cgrp stays accessible and break the
-         * active_ref protection.
-         */
-        cgroup_get(cgrp);
-        kernfs_break_active_protection(of->kn);
-
-        mutex_lock(&cgroup_tree_mutex);
-        if (!cgroup_lock_live_group(cgrp)) {
-                ret = -ENODEV;
-                goto out_unlock_tree;
-        }
+        cgrp = cgroup_kn_lock_live(of->kn);
+        if (!cgrp)
+                return -ENODEV;
 
         for_each_subsys(ss, ssid) {
                 if (enable & (1 << ssid)) {
@@ -2610,14 +2668,12 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                                 cgroup_get(child);
                                 prepare_to_wait(&child->offline_waitq, &wait,
                                                 TASK_UNINTERRUPTIBLE);
-                                mutex_unlock(&cgroup_mutex);
-                                mutex_unlock(&cgroup_tree_mutex);
+                                cgroup_kn_unlock(of->kn);
                                 schedule();
                                 finish_wait(&child->offline_waitq, &wait);
                                 cgroup_put(child);
 
-                                ret = restart_syscall();
-                                goto out_unbreak;
+                                return restart_syscall();
                         }
 
                         /* unavailable or not enabled on the parent? */
@@ -2693,12 +2749,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
         kernfs_activate(cgrp->kn);
         ret = 0;
 out_unlock:
-        mutex_unlock(&cgroup_mutex);
-out_unlock_tree:
-        mutex_unlock(&cgroup_tree_mutex);
-out_unbreak:
-        kernfs_unbreak_active_protection(of->kn);
-        cgroup_put(cgrp);
+        cgroup_kn_unlock(of->kn);
         return ret ?: nbytes;
 
 err_undo_css:
@@ -4238,25 +4289,16 @@ err_free_css:
 static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
                         umode_t mode)
 {
-        struct cgroup *parent = parent_kn->priv, *cgrp;
-        struct cgroup_root *root = parent->root;
+        struct cgroup *parent, *cgrp;
+        struct cgroup_root *root;
         struct cgroup_subsys *ss;
         struct kernfs_node *kn;
         int ssid, ret;
 
-        /*
-         * cgroup_mkdir() grabs cgroup_tree_mutex which nests outside
-         * kernfs active_ref and cgroup_create() already synchronizes
-         * properly against removal through cgroup_lock_live_group().
-         * Break it before calling cgroup_create().
-         */
-        cgroup_get(parent);
-        kernfs_break_active_protection(parent_kn);
-        mutex_lock(&cgroup_tree_mutex);
-        if (!cgroup_lock_live_group(parent)) {
-                ret = -ENODEV;
-                goto out_unlock_tree;
-        }
+        parent = cgroup_kn_lock_live(parent_kn);
+        if (!parent)
+                return -ENODEV;
+        root = parent->root;
 
         /* allocate the cgroup and its ID, 0 is reserved for the root */
         cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
@@ -4348,11 +4390,7 @@ out_free_id:
 out_free_cgrp:
         kfree(cgrp);
 out_unlock:
-        mutex_unlock(&cgroup_mutex);
-out_unlock_tree:
-        mutex_unlock(&cgroup_tree_mutex);
-        kernfs_unbreak_active_protection(parent_kn);
-        cgroup_put(parent);
+        cgroup_kn_unlock(parent_kn);
         return ret;
 
 out_destroy:
@@ -4579,32 +4617,17 @@ static void cgroup_destroy_css_killed(struct cgroup *cgrp)
 
 static int cgroup_rmdir(struct kernfs_node *kn)
 {
-        struct cgroup *cgrp = kn->priv;
+        struct cgroup *cgrp;
         int ret = 0;
 
-        /*
-         * This is self-destruction but @kn can't be removed while this
-         * callback is in progress.  Let's break active protection.  Once
-         * the protection is broken, @cgrp can be destroyed at any point.
-         * Pin it so that it stays accessible.
-         */
-        cgroup_get(cgrp);
-        kernfs_break_active_protection(kn);
-
-        mutex_lock(&cgroup_tree_mutex);
-        mutex_lock(&cgroup_mutex);
-
-        /*
-         * @cgrp might already have been destroyed while we're trying to
-         * grab the mutexes.
-         */
-        if (!cgroup_is_dead(cgrp))
-                ret = cgroup_destroy_locked(cgrp);
+        cgrp = cgroup_kn_lock_live(kn);
+        if (!cgrp)
+                return 0;
+        cgroup_get(cgrp);       /* for @kn->priv clearing */
 
-        mutex_unlock(&cgroup_mutex);
-        mutex_unlock(&cgroup_tree_mutex);
+        ret = cgroup_destroy_locked(cgrp);
 
-        kernfs_unbreak_active_protection(kn);
+        cgroup_kn_unlock(kn);
 
         /*
          * There are two control paths which try to determine cgroup from