path: root/kernel
diff options
authorTejun Heo <>2016-03-15 20:43:04 -0400
committerTejun Heo <>2016-03-16 13:31:46 -0700
commit2b021cbf3cb6208f0d40fd2f1869f237934340ed (patch)
treed5da3706354863607f2522cd0416e1de5993732f /kernel
parent6c83e6cb0ce897818878a7d3b1b25d5dc8f611a2 (diff)
cgroup: ignore css_sets associated with dead cgroups during migration
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 <> Fixes: 2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups") Cc: # v4.4+
Diffstat (limited to 'kernel')
1 files changed, 18 insertions, 2 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e22df5d81e59..d57318950076 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2516,6 +2516,14 @@ static void cgroup_migrate_add_src(struct css_set *src_cset,
+ /*
+ * 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))
@@ -5258,6 +5266,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
struct cgroup_subsys_state *css;
+ struct cgrp_cset_link *link;
int ssid;
@@ -5278,11 +5287,18 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
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)