lkml.org 
[lkml]   [2024]   [Apr]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous
From
On 4/3/24 08:02, Michal Koutný wrote:
> On Tue, Apr 02, 2024 at 11:30:11AM -0400, Waiman Long <longman@redhat.com> wrote:
>> Yes, there is a potential that a cpus_read_lock() may be called leading to
>> deadlock. So unless we reverse the current cgroup_mutex --> cpu_hotplug_lock
>> ordering, it is not safe to call cgroup_transfer_tasks() directly.
> I see that cgroup_transfer_tasks() has the only user -- cpuset. What
> about bending it for the specific use like:
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 34aaf0e87def..64deb7212c5c 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -109,7 +109,7 @@ struct cgroup *cgroup_get_from_fd(int fd);
> struct cgroup *cgroup_v1v2_get_from_fd(int fd);
>
> int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
> -int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
> +int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from);
>
> int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
> int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
> index 520a11cb12f4..f97025858c7a 100644
> --- a/kernel/cgroup/cgroup-v1.c
> +++ b/kernel/cgroup/cgroup-v1.c
> @@ -91,7 +91,8 @@ EXPORT_SYMBOL_GPL(cgroup_attach_task_all);
> *
> * Return: %0 on success or a negative errno code on failure
> */
> -int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
> +int cgroup_transfer_tasks_locked(struct cgroup *to, struct cgroup *from)
> {
> DEFINE_CGROUP_MGCTX(mgctx);
> struct cgrp_cset_link *link;
> @@ -106,9 +106,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
> if (ret)
> return ret;
>
> - cgroup_lock();
> -
> - cgroup_attach_lock(true);
> + /* The locking rules serve specific purpose of v1 cpuset hotplug
> + * migration, see hotplug_update_tasks_legacy() and
> + * cgroup_attach_lock() */
> + lockdep_assert_held(&cgroup_mutex);
> + lockdep_assert_cpus_held();
> + percpu_down_write(&cgroup_threadgroup_rwsem);
>
> /* all tasks in @from are being moved, all csets are source */
> spin_lock_irq(&css_set_lock);
> @@ -144,8 +146,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
> } while (task && !ret);
> out_err:
> cgroup_migrate_finish(&mgctx);
> - cgroup_attach_unlock(true);
> - cgroup_unlock();
> + percpu_up_write(&cgroup_threadgroup_rwsem);
> return ret;
> }
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 13d27b17c889..94fb8b26f038 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4331,7 +4331,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
> nodes_empty(parent->mems_allowed))
> parent = parent_cs(parent);
>
> - if (cgroup_transfer_tasks(parent->css.cgroup, cs->css.cgroup)) {
> + if (cgroup_transfer_tasks_locked(parent->css.cgroup, cs->css.cgroup)) {
> pr_err("cpuset: failed to transfer tasks out of empty cpuset ");
> pr_cont_cgroup_name(cs->css.cgroup);
> pr_cont("\n");
> @@ -4376,21 +4376,9 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
>
> /*
> * Move tasks to the nearest ancestor with execution resources,
> - * This is full cgroup operation which will also call back into
> - * cpuset. Execute it asynchronously using workqueue.
> */
> - if (is_empty && css_tryget_online(&cs->css)) {
> - struct cpuset_remove_tasks_struct *s;
> -
> - s = kzalloc(sizeof(*s), GFP_KERNEL);
> - if (WARN_ON_ONCE(!s)) {
> - css_put(&cs->css);
> - return;
> - }
> -
> - s->cs = cs;
> - INIT_WORK(&s->work, cpuset_migrate_tasks_workfn);
> - schedule_work(&s->work);
> + if (is_empty)
> + remove_tasks_in_empty_cpuset(cs);
> }
> }
>

It still won't work because of the possibility of mutiple tasks
involving in a circular locking dependency. The hotplug thread always
acquire the cpu_hotplug_lock first before acquiring cpuset_mutex or
cgroup_mtuex in this case (cpu_hotplug_lock --> cgroup_mutex). Other
tasks calling into cgroup code will acquire the pair in the order
cgroup_mutex --> cpu_hotplug_lock. This may lead to a deadlock if these
2 locking sequences happen at the same time. Lockdep will certainly
spill out a splat because of this. So unless we change all the relevant
cgroup code to the new cpu_hotplug_lock --> cgroup_mutex locking order,
the hotplug code can't call cgroup_transfer_tasks() directly.

Cheers,
Longman


\
 
 \ /
  Last update: 2024-05-27 16:22    [W:0.081 / U:0.044 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site