4 files changed, 79 insertions, 19 deletions

diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroups/cgroups.txt
index 93feb8444489..cdc46a501b85 100644
--- a/Documentation/cgroups/cgroups.txt
+++ b/Documentation/cgroups/cgroups.txt
@@ -476,11 +476,13 @@ cgroup->parent is still valid. (Note - can also be called for a
 newly-created cgroup if an error occurs after this subsystem's
 create() method has been called for the new cgroup).
 
-void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
+int pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
 
 Called before checking the reference count on each subsystem. This may
 be useful for subsystems which have some extra references even if
-there are not tasks in the cgroup.
+there are not tasks in the cgroup. If pre_destroy() returns error code,
+rmdir() will fail with it. From this behavior, pre_destroy() can be
+called multiple times against a cgroup.
 
 int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
                struct task_struct *task)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 9a23bb098205..7d824b80b3d7 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -135,6 +135,10 @@ enum {
         CGRP_RELEASABLE,
         /* Control Group requires release notifications to userspace */
         CGRP_NOTIFY_ON_RELEASE,
+        /*
+         * A thread in rmdir() is wating for this cgroup.
+         */
+        CGRP_WAIT_ON_RMDIR,
 };
 
 struct cgroup {
@@ -360,7 +364,7 @@ int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task);
 struct cgroup_subsys {
         struct cgroup_subsys_state *(*create)(struct cgroup_subsys *ss,
                                                   struct cgroup *cgrp);
-        void (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
+        int (*pre_destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
         void (*destroy)(struct cgroup_subsys *ss, struct cgroup *cgrp);
         int (*can_attach)(struct cgroup_subsys *ss,
                           struct cgroup *cgrp, struct task_struct *tsk);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d3c521137425..fc5e4a48582f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -622,13 +622,18 @@ static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
  * Call subsys's pre_destroy handler.
  * This is called before css refcnt check.
  */
-static void cgroup_call_pre_destroy(struct cgroup *cgrp)
+static int cgroup_call_pre_destroy(struct cgroup *cgrp)
 {
         struct cgroup_subsys *ss;
+        int ret = 0;
+
         for_each_subsys(cgrp->root, ss)
-                if (ss->pre_destroy)
-                        ss->pre_destroy(ss, cgrp);
-        return;
+                if (ss->pre_destroy) {
+                        ret = ss->pre_destroy(ss, cgrp);
+                        if (ret)
+                                break;
+                }
+        return ret;
 }
 
 static void free_cgroup_rcu(struct rcu_head *obj)
@@ -722,6 +727,22 @@ static void cgroup_d_remove_dir(struct dentry *dentry)
         remove_dir(dentry);
 }
 
+/*
+ * A queue for waiters to do rmdir() cgroup. A tasks will sleep when
+ * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some
+ * reference to css->refcnt. In general, this refcnt is expected to goes down
+ * to zero, soon.
+ *
+ * CGRP_WAIT_ON_RMDIR flag is modified under cgroup's inode->i_mutex;
+ */
+DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
+
+static void cgroup_wakeup_rmdir_waiters(const struct cgroup *cgrp)
+{
+        if (unlikely(test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
+                wake_up_all(&cgroup_rmdir_waitq);
+}
+
 static int rebind_subsystems(struct cgroupfs_root *root,
                               unsigned long final_bits)
 {
@@ -1317,6 +1338,12 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
         set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
         synchronize_rcu();
         put_css_set(cg);
+
+        /*
+         * wake up rmdir() waiter. the rmdir should fail since the cgroup
+         * is no longer empty.
+         */
+        cgroup_wakeup_rmdir_waiters(cgrp);
         return 0;
 }
 
@@ -2608,9 +2635,11 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
         struct cgroup *cgrp = dentry->d_fsdata;
         struct dentry *d;
         struct cgroup *parent;
+        DEFINE_WAIT(wait);
+        int ret;
 
         /* the vfs holds both inode->i_mutex already */
-
+again:
         mutex_lock(&cgroup_mutex);
         if (atomic_read(&cgrp->count) != 0) {
                 mutex_unlock(&cgroup_mutex);
@@ -2626,17 +2655,39 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
          * Call pre_destroy handlers of subsys. Notify subsystems
          * that rmdir() request comes.
          */
-        cgroup_call_pre_destroy(cgrp);
+        ret = cgroup_call_pre_destroy(cgrp);
+        if (ret)
+                return ret;
 
         mutex_lock(&cgroup_mutex);
         parent = cgrp->parent;
-
-        if (atomic_read(&cgrp->count)
-            || !list_empty(&cgrp->children)
-            || !cgroup_clear_css_refs(cgrp)) {
+        if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
                 mutex_unlock(&cgroup_mutex);
                 return -EBUSY;
         }
+        /*
+         * css_put/get is provided for subsys to grab refcnt to css. In typical
+         * case, subsystem has no reference after pre_destroy(). But, under
+         * hierarchy management, some *temporal* refcnt can be hold.
+         * To avoid returning -EBUSY to a user, waitqueue is used. If subsys
+         * is really busy, it should return -EBUSY at pre_destroy(). wake_up
+         * is called when css_put() is called and refcnt goes down to 0.
+         */
+        set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
+        prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
+
+        if (!cgroup_clear_css_refs(cgrp)) {
+                mutex_unlock(&cgroup_mutex);
+                schedule();
+                finish_wait(&cgroup_rmdir_waitq, &wait);
+                clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
+                if (signal_pending(current))
+                        return -EINTR;
+                goto again;
+        }
+        /* NO css_tryget() can success after here. */
+        finish_wait(&cgroup_rmdir_waitq, &wait);
+        clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
 
         spin_lock(&release_list_lock);
         set_bit(CGRP_REMOVED, &cgrp->flags);
@@ -3194,10 +3245,12 @@ void __css_put(struct cgroup_subsys_state *css)
 {
         struct cgroup *cgrp = css->cgroup;
         rcu_read_lock();
-        if ((atomic_dec_return(&css->refcnt) == 1) &&
-            notify_on_release(cgrp)) {
-                set_bit(CGRP_RELEASABLE, &cgrp->flags);
-                check_for_release(cgrp);
+        if (atomic_dec_return(&css->refcnt) == 1) {
+                if (notify_on_release(cgrp)) {
+                        set_bit(CGRP_RELEASABLE, &cgrp->flags);
+                        check_for_release(cgrp);
+                }
+                cgroup_wakeup_rmdir_waiters(cgrp);
         }
         rcu_read_unlock();
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8e4be9cb2a6a..8ffec674c5ac 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2272,11 +2272,12 @@ free_out:
         return ERR_PTR(-ENOMEM);
 }
 
-static void mem_cgroup_pre_destroy(struct cgroup_subsys *ss,
+static int mem_cgroup_pre_destroy(struct cgroup_subsys *ss,
                                         struct cgroup *cont)
 {
         struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
-        mem_cgroup_force_empty(mem, false);
+
+        return mem_cgroup_force_empty(mem, false);
 }
 
 static void mem_cgroup_destroy(struct cgroup_subsys *ss,