Patch "cgroup: Fix deadlock in cpu hotplug path" has been added to the 4.14-stable tree

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

cgroup: Fix deadlock in cpu hotplug path

to the 4.14-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-fix-deadlock-in-cpu-hotplug-path.patch
and it can be found in the queue-4.14 subdirectory.

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

From: Prateek Sood <[email protected]>
Date: Tue, 19 Dec 2017 12:56:57 +0530
Subject: cgroup: Fix deadlock in cpu hotplug path

From: Prateek Sood <[email protected]>

[ Upstream commit 116d2f7496c51b2e02e8e4ecdd2bdf5fb9d5a641 ]

Deadlock during cgroup migration from cpu hotplug path when a task T is being moved from source to destination cgroup.

kworker/0:0
cpuset_hotplug_workfn()
cpuset_hotplug_update_tasks()
hotplug_update_tasks_legacy()
remove_tasks_in_empty_cpuset()
cgroup_transfer_tasks() // stuck in iterator loop
cgroup_migrate()
cgroup_migrate_add_task()

In cgroup_migrate_add_task() it checks for PF_EXITING flag of task T. Task T will not migrate to destination cgroup. css_task_iter_start() will keep pointing to task T in loop waiting for task T cg_list node to be removed.

Task T
do_exit()
exit_signals() // sets PF_EXITING
exit_task_namespaces()
switch_task_namespaces()
free_nsproxy()
put_mnt_ns()
drop_collected_mounts()
namespace_unlock()
synchronize_rcu()
_synchronize_rcu_expedited()
schedule_work() // on cpu0 low priority worker pool wait_event() // waiting for work item to execute

Task T inserted a work item in the worklist of cpu0 low priority worker pool. It is waiting for expedited grace period work item to execute. This work item will only be executed once kworker/0:0 complete execution of cpuset_hotplug_workfn().

kworker/0:0 ==> Task T ==>kworker/0:0

In case of PF_EXITING task being migrated from source to destination cgroup, migrate next available task in source cgroup.

---
 kernel/cgroup/cgroup-v1.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -123,7 +123,11 @@ int cgroup_transfer_tasks(struct cgroup
 	 */
 	do {
 		css_task_iter_start(&from->self, 0, &it);
-		task = css_task_iter_next(&it);
+
+		do {
+			task = css_task_iter_next(&it);
+		} while (task && (task->flags & PF_EXITING));
+
 		if (task)
 			get_task_struct(task);
 		css_task_iter_end(&it);

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

queue-4.14/cgroup-fix-deadlock-in-cpu-hotplug-path.patch