From 116d2f7496c51b2e02e8e4ecdd2bdf5fb9d5a641 Mon Sep 17 00:00:00 2001 From: Prateek Sood Date: Tue, 19 Dec 2017 12:56:57 +0530 Subject: 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. Signed-off-by: Prateek Sood Signed-off-by: Tejun Heo --- kernel/cgroup/cgroup-v1.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c index 024085daab1a..a2c05d2476ac 100644 --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -123,7 +123,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) */ 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); -- cgit v1.2.3