[Devel] [PATCH RHEL7 v16 12/13] ve/cgroup: added release_agent to each container root cgroup.

[Valeriy Vdovin]

Each container will now have access to it's own cgroup release_agent
file.
Creation:
  Normally all cgroup files are created during a call to cgroup_create
  by cgroup_populate_dir function. It creates or not creates all cgroup
  files once and they immediately become visible to userspace as
  filesystem objects.
  Due to specifics of container creation process, it is not possible to
  use the same code for 'release_agent' file creation. For VE to start
  operating, first a list of ordinary cgroups is being created for
  each subsystem, then the set of newly created cgroups are converted to
  "virtual roots", so at the time when cgroup_create is executed, there
  is no knowledge of wheather or not "release_agent" file should be
  created. This information only comes at "convertion" step which is
  'cgroup_mark_ve_roots' function. As the file is created dynamically
  in a live cgroup, a rather delicate locking sequence is present in
  the new code:
    - each new "virtual root" cgroup will have to add "release_agent" file,
      thus each cgroup's directory would need to be locked during
      the insertion time by cgroup->dentry->d_inode->i_mutex.
    - d_inode->i_mutex has an ordering dependency with cgroup_mutex
      (see cgroup_mount/cgroup_remount). They can not be locked in order
      {lock(cgroup_mutex), lock(inode->i_mutex)}.
    - to collect a list of cgroups, that need to become virtual we need
      cgroup_mutex lock to iterate active roots.
    - to overcome the above conflict we first need to collect a list of
      all virtual cgroups under cgroup_mutex lock, then release it and
      after that to insert "release_agent" to each root under
      inode->i_mutex lock.
    - to collect a list of cgroups on stack we utilize
      cgroup->cft_q_node, made specially for that purpose under it's own
      cgroup_cft_mutex.

Destruction:
  Destruction is done in reverse from the above within
  cgroup_unmark_ve_root.
  After file destruction we must prevent further write operations to
  this file in case when someone has opened this file prior to VE
  and cgroup destruction. This is achieved by checking if cgroup
  in the argument to cgroup_file_write function has features of host
  or virtual root.

https://jira.sw.ru/browse/PSBM-83887

Signed-off-by: Valeriy Vdovin <valeriy.vdovin at virtuozzo.com>
---
 include/linux/cgroup.h |  2 +-
 kernel/cgroup.c        | 79 ++++++++++++++++++++++++++++++++++++++++++++++----
 kernel/ve/ve.c         |  6 +++-
 3 files changed, 80 insertions(+), 7 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 37765f5..911dd48 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -636,7 +636,7 @@ int cgroup_task_count(const struct cgroup *cgrp);
 void cgroup_release_agent(struct work_struct *work);
 
 #ifdef CONFIG_VE
-void cgroup_mark_ve_roots(struct ve_struct *ve);
+int cgroup_mark_ve_roots(struct ve_struct *ve);
 void cgroup_unmark_ve_roots(struct ve_struct *ve);
 struct ve_struct *cgroup_get_ve_owner(struct cgroup *cgrp);
 #endif
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3f21ed2..9f8c8db 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2338,13 +2338,28 @@ static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
 	if (!cgroup_lock_live_group(cgrp))
 		return -ENODEV;
 
+	/*
+	 * Call to cgroup_get_local_root is a way to make sure
+	 * that cgrp in the argument is valid "virtual root"
+	 * cgroup. If it's not - this means that cgroup_unmark_ve_roots
+	 * has already cleared CGRP_VE_ROOT flag from this cgroup while
+	 * the file was still opened by other process and
+	 * that we shouldn't allow further writings via that file.
+	 */
 	root_cgrp = cgroup_get_local_root(cgrp);
 	BUG_ON(!root_cgrp);
+
+	if (root_cgrp != cgrp) {
+		ret = -EPERM;
+		goto out;
+	}
+
 	if (root_cgrp->ve_owner) {
 		ret = ve_set_release_agent_path(root_cgrp->ve_owner,
 			root_cgrp, buffer);
 	}
 
+out:
 	mutex_unlock(&cgroup_mutex);
 	return ret;
 }
@@ -4183,7 +4198,7 @@ static struct cftype files[] = {
 	},
 	{
 		.name = "release_agent",
-		.flags = CFTYPE_ONLY_ON_ROOT,
+		.flags = CFTYPE_ONLY_ON_ROOT | CFTYPE_VE_WRITABLE,
 		.read_seq_string = cgroup_release_agent_show,
 		.write_string = cgroup_release_agent_write,
 		.max_write_len = PATH_MAX,
@@ -4312,37 +4327,91 @@ static int subgroups_count(struct cgroup *cgroup)
 	return cgrps_count;
 }
 
+static struct cftype *get_cftype_by_name(const char *name)
+{
+	struct cftype *cft;
+	for (cft = files; cft->name[0] != '\0'; cft++) {
+		if (!strcmp(cft->name, name))
+			return cft;
+	}
+	return NULL;
+}
+
 #ifdef CONFIG_VE
-void cgroup_mark_ve_roots(struct ve_struct *ve)
+int cgroup_mark_ve_roots(struct ve_struct *ve)
 {
-	struct cgroup *cgrp;
+	struct cgroup *cgrp, *tmp;
 	struct cgroupfs_root *root;
+	int err = 0;
+	struct cftype *cft;
+	LIST_HEAD(pending);
+
+	cft = get_cftype_by_name("release_agent");
+	BUG_ON(!cft);
 
+	mutex_lock(&cgroup_cft_mutex);
 	mutex_lock(&cgroup_mutex);
 	for_each_active_root(root) {
 		cgrp = task_cgroup_from_root(ve->init_task, root);
 		rcu_assign_pointer(cgrp->ve_owner, ve);
 		set_bit(CGRP_VE_ROOT, &cgrp->flags);
+		dget(cgrp->dentry);
+		list_add_tail(&cgrp->cft_q_node, &pending);
 		if (test_bit(cpu_cgroup_subsys_id, &root->subsys_mask))
 			link_ve_root_cpu_cgroup(cgrp);
 	}
 	mutex_unlock(&cgroup_mutex);
+	list_for_each_entry_safe(cgrp, tmp, &pending, cft_q_node) {
+		struct inode *inode = cgrp->dentry->d_inode;
+
+		if (err) {
+			dput(cgrp->dentry);
+			continue;
+		}
+
+		mutex_lock(&inode->i_mutex);
+		mutex_lock(&cgroup_mutex);
+		if (!cgroup_is_removed(cgrp))
+			err = cgroup_add_file(cgrp, NULL, cft);
+		mutex_unlock(&cgroup_mutex);
+		mutex_unlock(&inode->i_mutex);
+
+		list_del_init(&cgrp->cft_q_node);
+		dput(cgrp->dentry);
+	}
+	mutex_unlock(&cgroup_cft_mutex);
+	return err;
 }
 
 void cgroup_unmark_ve_roots(struct ve_struct *ve)
 {
-	struct cgroup *cgrp;
+	struct cgroup *cgrp, *tmp;
 	struct cgroupfs_root *root;
+	struct cftype *cft;
+	LIST_HEAD(pending);
+	cft = get_cftype_by_name("release_agent");
 
+	mutex_lock(&cgroup_cft_mutex);
 	mutex_lock(&cgroup_mutex);
 	for_each_active_root(root) {
 		cgrp = css_cgroup_from_root(ve->root_css_set, root);
+		dget(cgrp->dentry);
+		list_add_tail(&cgrp->cft_q_node, &pending);
+	}
+	mutex_unlock(&cgroup_mutex);
+	list_for_each_entry_safe(cgrp, tmp, &pending, cft_q_node) {
+		struct inode *inode = cgrp->dentry->d_inode;
+		mutex_lock(&inode->i_mutex);
+		mutex_lock(&cgroup_mutex);
+		cgroup_rm_file(cgrp, cft);
 		BUG_ON(!rcu_dereference_protected(cgrp->ve_owner,
 				lockdep_is_held(&cgroup_mutex)));
 		rcu_assign_pointer(cgrp->ve_owner, NULL);
 		clear_bit(CGRP_VE_ROOT, &cgrp->flags);
+		mutex_unlock(&cgroup_mutex);
+		mutex_unlock(&inode->i_mutex);
 	}
-	mutex_unlock(&cgroup_mutex);
+	mutex_unlock(&cgroup_cft_mutex);
 	/* ve_owner == NULL will be visible */
 	synchronize_rcu();
 
diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index a12e021..5371c10 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -704,7 +704,9 @@ static int ve_start_container(struct ve_struct *ve)
 	if (err < 0)
 		goto err_iterate;
 
-	cgroup_mark_ve_roots(ve);
+	err = cgroup_mark_ve_roots(ve);
+	if (err)
+		goto err_mark_ve;
 
 	ve->is_running = 1;
 
@@ -714,6 +716,8 @@ static int ve_start_container(struct ve_struct *ve)
 
 	return 0;
 
+err_mark_ve:
+	ve_hook_iterate_fini(VE_SS_CHAIN, ve);
 err_iterate:
 	ve_workqueue_stop(ve);
 err_workqueue:
-- 
1.8.3.1