cgroup: convert cgroup_ida to cgroup_idr

This enables us to lookup a cgroup by its id.

v4:
- add a comment for idr_remove() in cgroup_offline_fn().

v3:
- on success, idr_alloc() returns the id but not 0, so fix the BUG_ON()
  in cgroup_init().
- pass the right value to idr_alloc() so that the id for dummy cgroup is 0.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 29 insertions, 8 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 00a7e07a1567..cca570e188fb 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -161,7 +161,7 @@ struct cgroup_name {
 struct cgroup {
         unsigned long flags;            /* "unsigned long" so bitops work */
 
-        int id;                         /* ida allocated in-hierarchy ID */
+        int id;                         /* idr allocated in-hierarchy ID */
 
         /*
          * We link our 'sibling' struct into our parent's 'children'.
@@ -322,7 +322,7 @@ struct cgroupfs_root {
         unsigned long flags;
 
         /* IDs for cgroups in this hierarchy */
-        struct ida cgroup_ida;
+        struct idr cgroup_idr;
 
         /* The path to use for release notifications. */
         char release_agent_path[PATH_MAX];
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 9577bebe2546..3f6593333525 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -866,8 +866,6 @@ static void cgroup_free_fn(struct work_struct *work)
          */
         dput(cgrp->parent->dentry);
 
-        ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
-
         /*
          * Drop the active superblock reference that we took when we
          * created the cgroup. This will free cgrp->root, if we are
@@ -1379,6 +1377,7 @@ static void init_cgroup_root(struct cgroupfs_root *root)
         cgrp->root = root;
         RCU_INIT_POINTER(cgrp->name, &root_cgroup_name);
         init_cgroup_housekeeping(cgrp);
+        idr_init(&root->cgroup_idr);
 }
 
 static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
@@ -1451,7 +1450,6 @@ static struct cgroupfs_root *cgroup_root_from_opts(struct cgroup_sb_opts *opts)
          */
         root->subsys_mask = opts->subsys_mask;
         root->flags = opts->flags;
-        ida_init(&root->cgroup_ida);
         if (opts->release_agent)
                 strcpy(root->release_agent_path, opts->release_agent);
         if (opts->name)
@@ -1467,7 +1465,7 @@ static void cgroup_free_root(struct cgroupfs_root *root)
                 /* hierarhcy ID shoulid already have been released */
                 WARN_ON_ONCE(root->hierarchy_id);
 
-                ida_destroy(&root->cgroup_ida);
+                idr_destroy(&root->cgroup_idr);
                 kfree(root);
         }
 }
@@ -1582,6 +1580,11 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                 mutex_lock(&cgroup_mutex);
                 mutex_lock(&cgroup_root_mutex);
 
+                root_cgrp->id = idr_alloc(&root->cgroup_idr, root_cgrp,
+                                           0, 1, GFP_KERNEL);
+                if (root_cgrp->id < 0)
+                        goto unlock_drop;
+
                 /* Check for name clashes with existing mounts */
                 ret = -EBUSY;
                 if (strlen(root->name))
@@ -4253,7 +4256,11 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 goto err_free_cgrp;
         rcu_assign_pointer(cgrp->name, name);
 
-        cgrp->id = ida_simple_get(&root->cgroup_ida, 1, 0, GFP_KERNEL);
+        /*
+         * Temporarily set the pointer to NULL, so idr_find() won't return
+         * a half-baked cgroup.
+         */
+        cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
         if (cgrp->id < 0)
                 goto err_free_name;
 
@@ -4351,6 +4358,8 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                 }
         }
 
+        idr_replace(&root->cgroup_idr, cgrp, cgrp->id);
+
         err = cgroup_addrm_files(cgrp, NULL, cgroup_base_files, true);
         if (err)
                 goto err_destroy;
@@ -4377,7 +4386,7 @@ err_free_all:
         /* Release the reference count that we took on the superblock */
         deactivate_super(sb);
 err_free_id:
-        ida_simple_remove(&root->cgroup_ida, cgrp->id);
+        idr_remove(&root->cgroup_idr, cgrp->id);
 err_free_name:
         kfree(rcu_dereference_raw(cgrp->name));
 err_free_cgrp:
@@ -4570,6 +4579,14 @@ static void cgroup_offline_fn(struct work_struct *work)
         /* delete this cgroup from parent->children */
         list_del_rcu(&cgrp->sibling);
 
+        /*
+         * We should remove the cgroup object from idr before its grace
+         * period starts, so we won't be looking up a cgroup while the
+         * cgroup is being freed.
+         */
+        idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
+        cgrp->id = -1;
+
         dput(d);
 
         set_bit(CGRP_RELEASABLE, &parent->flags);
@@ -4895,6 +4912,10 @@ int __init cgroup_init(void)
 
         BUG_ON(cgroup_init_root_id(&cgroup_dummy_root, 0, 1));
 
+        err = idr_alloc(&cgroup_dummy_root.cgroup_idr, cgroup_dummy_top,
+                        0, 1, GFP_KERNEL);
+        BUG_ON(err < 0);
+
         mutex_unlock(&cgroup_root_mutex);
         mutex_unlock(&cgroup_mutex);