Patch "cgroup: ignore css_sets associated with dead cgroups during migration" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

cgroup: ignore css_sets associated with dead cgroups during migration

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cgroup-ignore-css_sets-associated-with-dead-cgroups-during-migration.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Tue, 15 Mar 2016 20:43:04 -0400
Subject: cgroup: ignore css_sets associated with dead cgroups during migration

From: Tejun Heo <[email protected]>

commit 2b021cbf3cb6208f0d40fd2f1869f237934340ed upstream.

Before 2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups"), all dead tasks were associated with init_css_set. If a zombie task is requested for migration, while migration prep operations would still be performed on init_css_set, the actual migration would ignore zombie tasks. As init_css_set is always valid, this worked fine.

However, after 2e91fa7f6d45, zombie tasks stay with the css_set it was associated with at the time of death. Let's say a task T associated with cgroup A on hierarchy H-1 and cgroup B on hiearchy H-2. After T becomes a zombie, it would still remain associated with A and B. If A only contains zombie tasks, it can be removed. On removal, A gets marked offline but stays pinned until all zombies are drained. At this point, if migration is initiated on T to a cgroup C on hierarchy H-2, migration path would try to prepare T's css_set for migration and trigger the following.

 WARNING: CPU: 0 PID: 1576 at kernel/cgroup.c:474 cgroup_get+0x121/0x160()
 CPU: 0 PID: 1576 Comm: bash Not tainted 4.4.0-work+ #289
 ...
 Call Trace:
  [<ffffffff8127e63c>] dump_stack+0x4e/0x82
  [<ffffffff810445e8>] warn_slowpath_common+0x78/0xb0
  [<ffffffff810446d5>] warn_slowpath_null+0x15/0x20
  [<ffffffff810c33e1>] cgroup_get+0x121/0x160
  [<ffffffff810c349b>] link_css_set+0x7b/0x90
  [<ffffffff810c4fbc>] find_css_set+0x3bc/0x5e0
  [<ffffffff810c5269>] cgroup_migrate_prepare_dst+0x89/0x1f0
  [<ffffffff810c7547>] cgroup_attach_task+0x157/0x230
  [<ffffffff810c7a17>] __cgroup_procs_write+0x2b7/0x470
  [<ffffffff810c7bdc>] cgroup_tasks_write+0xc/0x10
  [<ffffffff810c4790>] cgroup_file_write+0x30/0x1b0
  [<ffffffff811c68fc>] kernfs_fop_write+0x13c/0x180
  [<ffffffff81151673>] __vfs_write+0x23/0xe0
  [<ffffffff81152494>] vfs_write+0xa4/0x1a0
  [<ffffffff811532d4>] SyS_write+0x44/0xa0
  [<ffffffff814af2d7>] entry_SYSCALL_64_fastpath+0x12/0x6f

It doesn't make sense to prepare migration for css_sets pointing to dead cgroups as they are guaranteed to contain only zombies which are ignored later during migration. This patch makes cgroup destruction path mark all affected css_sets as dead and updates the migration path to ignore them during preparation.

---
 include/linux/cgroup-defs.h |    3 +++
 kernel/cgroup.c             |   20 ++++++++++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

@@ -210,6 +210,9 @@ struct css_set {
 	/* all css_task_iters currently walking this cset */
 	struct list_head task_iters;
 
+	/* dead and being drained, ignore for migration */
+	bool dead;
+
 	/* For RCU-protected deletion */
 	struct rcu_head rcu_head;
 };

@@ -2474,6 +2474,14 @@ static void cgroup_migrate_add_src(struc
 	lockdep_assert_held(&cgroup_mutex);
 	lockdep_assert_held(&css_set_lock);
 
+	/*
+	 * If ->dead, @src_set is associated with one or more dead cgroups
+	 * and doesn't contain any migratable tasks.  Ignore it early so
+	 * that the rest of migration path doesn't get confused by it.
+	 */
+	if (src_cset->dead)
+		return;
+
 	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
 
 	if (!list_empty(&src_cset->mg_preload_node))

@@ -5114,6 +5122,7 @@ static int cgroup_destroy_locked(struct
 	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
 	struct cgroup_subsys_state *css;
+	struct cgrp_cset_link *link;
 	int ssid;
 
 	lockdep_assert_held(&cgroup_mutex);

@@ -5134,11 +5143,18 @@ static int cgroup_destroy_locked(struct
 		return -EBUSY;
 
 	/*
-	 * Mark @cgrp dead.  This prevents further task migration and child
-	 * creation by disabling cgroup_lock_live_group().
+	 * Mark @cgrp and the associated csets dead.  The former prevents
+	 * further task migration and child creation by disabling
+	 * cgroup_lock_live_group().  The latter makes the csets ignored by
+	 * the migration path.
 	 */
 	cgrp->self.flags &= ~CSS_ONLINE;
 
+	spin_lock_bh(&css_set_lock);
+	list_for_each_entry(link, &cgrp->cset_links, cset_link)
+		link->cset->dead = true;
+	spin_unlock_bh(&css_set_lock);
+
 	/* initiate massacre of all css's */
 	for_each_css(css, ssid, cgrp)
 		kill_css(css);

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/cgroup-ignore-css_sets-associated-with-dead-cgroups-during-migration.patch