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 4.4-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-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 2b021cbf3cb6208f0d40fd2f1869f237934340ed Mon Sep 17 00:00:00 2001 From: Tejun Heo <tj@xxxxxxxxxx> Date: Tue, 15 Mar 2016 20:43:04 -0400 Subject: cgroup: ignore css_sets associated with dead cgroups during migration From: Tejun Heo <tj@xxxxxxxxxx> 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. Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> Fixes: 2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups") Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- include/linux/cgroup-defs.h | 3 +++ kernel/cgroup.c | 20 ++++++++++++++++++-- 2 files changed, 21 insertions(+), 2 deletions(-) --- a/include/linux/cgroup-defs.h +++ b/include/linux/cgroup-defs.h @@ -216,6 +216,9 @@ 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; }; --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2498,6 +2498,14 @@ 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)) @@ -5131,6 +5139,7 @@ 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); @@ -5151,11 +5160,18 @@ 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 tj@xxxxxxxxxx are queue-4.4/cgroup-ignore-css_sets-associated-with-dead-cgroups-during-migration.patch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html