From: Prateek Sood prsood@codeaurora.org

commit 116d2f7496c51b2e02e8e4ecdd2bdf5fb9d5a641 upstream.

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 prsood@codeaurora.org Signed-off-by: Tejun Heo tj@kernel.org [AmitP: Upstream commit cherry-pick failed, so I picked the backported changes from CAF/msm-4.9 tree instead: https://source.codeaurora.org/quic/la/kernel/msm-4.9/commit/?id=49b74f169641...] Signed-off-by: Amit Pundir amit.pundir@linaro.org --- This patch can be cleanly applied and build tested on 4.4.y and 3.18.y as well but I couldn't find it in msm-4.4 and msm-3.18 trees. So this patch is really untested on those stable trees. Build tested on 4.9.131, 4.4.159 and 3.18.123 for ARCH=arm/arm64 allmodconfig.

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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 4c233437ee1a..bb0cf1caf1cd 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -4386,7 +4386,11 @@ 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);