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Subject[patch] cgroups: disallow attaching kthreadd
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The hazzards of moving kthreadd into a non-root cgroup is still present
in mainline. Last go 'round stalled with Peter not liking the
cpuset,cpu per controller specific exclusion. I agree that total
exclusion is the better option, and below is a respin doing that.

cgroups: disallow attaching kthreadd

Allowing kthreadd to be moved to a non-root group makes no sense, it being
a global resource, and needlessly leads unsuspecting users toward trouble.

1. An RT workqueue worker thread spawned in a task group with no rt_runtime
allocated is not schedulable. Simple user error, but harmful to the box.

2. A worker thread which acquires PF_THREAD_BOUND can never leave a cpuset,
rendering the cpuset immortal.

Save the user some unexpected trouble, just say no.

Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
kernel/cgroup.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -60,6 +60,7 @@
#include <linux/eventfd.h>
#include <linux/poll.h>
#include <linux/flex_array.h> /* used in cgroup_attach_proc */
+#include <linux/kthread.h>

#include <linux/atomic.h>

@@ -1894,6 +1895,14 @@ int cgroup_attach_task(struct cgroup *cg
if (tsk->flags & PF_EXITING)
return -ESRCH;

+ /*
+ * Workqueue threads may acquire PF_THREAD_BOUND and become
+ * trapped in a cpuset, or RT worker may be born in a cgroup
+ * with no rt_runtime allocated. Just say no.
+ */
+ if (tsk == kthreadd_task)
+ return -EINVAL;
+
/* Nothing to do if the task is already in that cgroup */
oldcgrp = task_cgroup_from_root(tsk, root);
if (cgrp == oldcgrp)
@@ -2172,6 +2181,17 @@ static int attach_task_by_pid(struct cgr

if (threadgroup)
tsk = tsk->group_leader;
+
+ /*
+ * Workqueue threads may acquire PF_THREAD_BOUND and become
+ * trapped in a cpuset, or RT worker may be born in a cgroup
+ * with no rt_runtime allocated. Just say no.
+ */
+ if (tsk == kthreadd_task) {
+ ret = -EINVAL;
+ goto out_unlock_cgroup;
+ }
+
get_task_struct(tsk);
rcu_read_unlock();





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