From 47d4272073b20d8a359b4279a9eb468ea2541289 Mon Sep 17 00:00:00 2001 From: Prateek Sood Date: Tue, 19 Dec 2017 16:11:54 +0530 Subject: [PATCH] cgroup: Fix deadlock in cpu hotplug path 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. Change-Id: I706aeff24c336c660b5a9b3e5a409c5769968960 Signed-off-by: Prateek Sood --- kernel/cgroup.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 3fdb7545852e..cd3d81961cc2 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -4092,7 +4092,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) */ do { css_task_iter_start(&from->self, &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); -- 2.11.0