Re: [PATCH] cgroup/cpuset: fix circular locking dependency
From: Prateek Sood <hidden>
Date: 2017-12-13 14:28:41
Also in:
lkml
Subsystem:
control group (cgroup), the rest · Maintainers:
Tejun Heo, Johannes Weiner, Michal Koutný, Linus Torvalds
On 12/11/2017 09:02 PM, Tejun Heo wrote:
Hello, Prateek. On Fri, Dec 08, 2017 at 05:15:55PM +0530, Prateek Sood wrote:quoted
There is one deadlock issue 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.Heh, that's a bug in cgroup_transfer_tasks() which happened because I forgot to update when we changed how we handle exiting tasks. The right thing to do here is making cgroup_transfer_tasks() repeat iff there were a valid migration target which didn't get transferred. Thanks.
Hi TJ, Did you mean something like below. If not then could you please share a patch for this problem in cgroup_transfer_tasks().
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 473e0c0..41de618 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h@@ -143,6 +143,8 @@ struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset, void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags, struct css_task_iter *it); +void css_task_migrate_iter_start(struct cgroup_subsys_state *css, + unsigned int flags, struct css_task_iter *it); struct task_struct *css_task_iter_next(struct css_task_iter *it); void css_task_iter_end(struct css_task_iter *it);
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 024085d..12279ae 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c@@ -122,7 +122,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from) * ->can_attach() fails. */ do { - css_task_iter_start(&from->self, 0, &it); + css_task_migrate_iter_start(&from->self, 0, &it); task = css_task_iter_next(&it); if (task) get_task_struct(task);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 0b1ffe1..3c1d2d2 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c@@ -4189,6 +4189,42 @@ void css_task_iter_start(struct cgroup_subsys_state *css, unsigned int flags, spin_unlock_irq(&css_set_lock); } +void css_task_migrate_iter_start(struct cgroup_subsys_state *css, + unsigned int flags, struct css_task_iter *it) +{ + struct task_struct *task = NULL; + /* no one should try to iterate before mounting cgroups */ + WARN_ON_ONCE(!use_task_css_set_links); + + memset(it, 0, sizeof(*it)); + + spin_lock_irq(&css_set_lock); + + it->ss = css->ss; + it->flags = flags; + + if (it->ss) + it->cset_pos = &css->cgroup->e_csets[css->ss->id]; + else + it->cset_pos = &css->cgroup->cset_links; + + it->cset_head = it->cset_pos; + + css_task_iter_advance_css_set(it); + + while (it->task_pos) { + task = list_entry(it->task_pos, struct task_struct, + cg_list); + + if (likely(!(task->flags & PF_EXITING))) + break; + + css_task_iter_advance(it); + } + + spin_unlock_irq(&css_set_lock); +} + /** * css_task_iter_next - return the next task for the iterator * @it: the task iterator being iterated
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