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Message-ID: <7e62b37d-6c9c-4e55-a01a-175695475cb5@redhat.com>
Date: Wed, 3 Apr 2024 09:38:07 -0400
From: Waiman Long <longman@...hat.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
Johannes Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki"
<rafael@...nel.org>, Len Brown <len.brown@...el.com>,
Pavel Machek <pavel@....cz>, Shuah Khan <shuah@...nel.org>,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
linux-pm@...r.kernel.org, linux-kselftest@...r.kernel.org,
Frederic Weisbecker <frederic@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Valentin Schneider <vschneid@...hat.com>,
Anna-Maria Behnsen <anna-maria@...utronix.de>, Alex Shi <alexs@...nel.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing
synchronous
On 4/3/24 08:02, Michal Koutný wrote:
> On Tue, Apr 02, 2024 at 11:30:11AM -0400, Waiman Long <longman@...hat.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
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