cgroup: introduce struct cfent

This patch adds cfent (cgroup file entry) which is the association
between a cgroup and a file.  This is in-cgroup representation of
files under a cgroup directory.  This simplifies walking walking
cgroup files and thus cgroup_clear_directory(), which is now
implemented in two parts - cgroup_rm_file() and a loop around it.

cgroup_rm_file() will be used to implement cftype removal and cfent is
scheduled to serve cgroup specific per-file data (e.g. for sysfs-like
"sever" semantics).

v2: - cfe was freed from cgroup_rm_file() which led to use-after-free
      if the file had openers at the time of removal.  Moved to
      cgroup_diput().

    - cgroup_clear_directory() triggered WARN_ON_ONCE() if d_subdirs
      wasn't empty after removing all files.  This triggered
      spuriously if some files were open during directory clearing.
      Removed.

v3: - In cgroup_diput(), WARN_ONCE(!list_empty(&cfe->node)) could be
      spuriously triggered for root cgroups because they don't go
      through cgroup_clear_directory() on unmount.  Don't trigger WARN
      for root cgroups.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Glauber Costa <glommer@parallels.com>

2 files changed, 78 insertions, 36 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 7a3d96755114..87b034ed0c31 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -175,6 +175,7 @@ struct cgroup {
          */
         struct list_head sibling;       /* my parent's children */
         struct list_head children;      /* my children */
+        struct list_head files;         /* my files */
 
         struct cgroup *parent;          /* my parent */
         struct dentry __rcu *dentry;    /* cgroup fs entry, RCU protected */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2ab29eb381b7..7d5d1c927d9d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -148,6 +148,15 @@ struct cgroupfs_root {
 static struct cgroupfs_root rootnode;
 
 /*
+ * cgroupfs file entry, pointed to from leaf dentry->d_fsdata.
+ */
+struct cfent {
+        struct list_head                node;
+        struct dentry                   *dentry;
+        struct cftype                   *type;
+};
+
+/*
  * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
  * cgroup_subsys->use_id != 0.
  */
@@ -287,11 +296,16 @@ static inline struct cgroup *__d_cgrp(struct dentry *dentry)
         return dentry->d_fsdata;
 }
 
-static inline struct cftype *__d_cft(struct dentry *dentry)
+static inline struct cfent *__d_cfe(struct dentry *dentry)
 {
         return dentry->d_fsdata;
 }
 
+static inline struct cftype *__d_cft(struct dentry *dentry)
+{
+        return __d_cfe(dentry)->type;
+}
+
 /* the list of cgroups eligible for automatic release. Protected by
  * release_list_lock */
 static LIST_HEAD(release_list);
@@ -877,6 +891,14 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
                 BUG_ON(!list_empty(&cgrp->pidlists));
 
                 kfree_rcu(cgrp, rcu_head);
+        } else {
+                struct cfent *cfe = __d_cfe(dentry);
+                struct cgroup *cgrp = dentry->d_parent->d_fsdata;
+
+                WARN_ONCE(!list_empty(&cfe->node) &&
+                          cgrp != &cgrp->root->top_cgroup,
+                          "cfe still linked for %s\n", cfe->type->name);
+                kfree(cfe);
         }
         iput(inode);
 }
@@ -895,34 +917,36 @@ static void remove_dir(struct dentry *d)
         dput(parent);
 }
 
-static void cgroup_clear_directory(struct dentry *dentry)
-{
-        struct list_head *node;
-
-        BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
-        spin_lock(&dentry->d_lock);
-        node = dentry->d_subdirs.next;
-        while (node != &dentry->d_subdirs) {
-                struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
-
-                spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
-                list_del_init(node);
-                if (d->d_inode) {
-                        /* This should never be called on a cgroup
-                         * directory with child cgroups */
-                        BUG_ON(d->d_inode->i_mode & S_IFDIR);
-                        dget_dlock(d);
-                        spin_unlock(&d->d_lock);
-                        spin_unlock(&dentry->d_lock);
-                        d_delete(d);
-                        simple_unlink(dentry->d_inode, d);
-                        dput(d);
-                        spin_lock(&dentry->d_lock);
-                } else
-                        spin_unlock(&d->d_lock);
-                node = dentry->d_subdirs.next;
+static int cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
+{
+        struct cfent *cfe;
+
+        lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
+        lockdep_assert_held(&cgroup_mutex);
+
+        list_for_each_entry(cfe, &cgrp->files, node) {
+                struct dentry *d = cfe->dentry;
+
+                if (cft && cfe->type != cft)
+                        continue;
+
+                dget(d);
+                d_delete(d);
+                simple_unlink(d->d_inode, d);
+                list_del_init(&cfe->node);
+                dput(d);
+
+                return 0;
         }
-        spin_unlock(&dentry->d_lock);
+        return -ENOENT;
+}
+
+static void cgroup_clear_directory(struct dentry *dir)
+{
+        struct cgroup *cgrp = __d_cgrp(dir);
+
+        while (!list_empty(&cgrp->files))
+                cgroup_rm_file(cgrp, NULL);
 }
 
 /*
@@ -1352,6 +1376,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 {
         INIT_LIST_HEAD(&cgrp->sibling);
         INIT_LIST_HEAD(&cgrp->children);
+        INIT_LIST_HEAD(&cgrp->files);
         INIT_LIST_HEAD(&cgrp->css_sets);
         INIT_LIST_HEAD(&cgrp->release_list);
         INIT_LIST_HEAD(&cgrp->pidlists);
@@ -2619,7 +2644,9 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                            const struct cftype *cft)
 {
         struct dentry *dir = cgrp->dentry;
+        struct cgroup *parent = __d_cgrp(dir);
         struct dentry *dentry;
+        struct cfent *cfe;
         int error;
         umode_t mode;
         char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
@@ -2635,17 +2662,31 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                 strcat(name, ".");
         }
         strcat(name, cft->name);
+
         BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));
+
+        cfe = kzalloc(sizeof(*cfe), GFP_KERNEL);
+        if (!cfe)
+                return -ENOMEM;
+
         dentry = lookup_one_len(name, dir, strlen(name));
-        if (!IS_ERR(dentry)) {
-                mode = cgroup_file_mode(cft);
-                error = cgroup_create_file(dentry, mode | S_IFREG,
-                                                cgrp->root->sb);
-                if (!error)
-                        dentry->d_fsdata = (void *)cft;
-                dput(dentry);
-        } else
+        if (IS_ERR(dentry)) {
                 error = PTR_ERR(dentry);
+                goto out;
+        }
+
+        mode = cgroup_file_mode(cft);
+        error = cgroup_create_file(dentry, mode | S_IFREG, cgrp->root->sb);
+        if (!error) {
+                cfe->type = (void *)cft;
+                cfe->dentry = dentry;
+                dentry->d_fsdata = cfe;
+                list_add_tail(&cfe->node, &parent->files);
+                cfe = NULL;
+        }
+        dput(dentry);
+out:
+        kfree(cfe);
         return error;
 }