Messages in this thread | | | Date | Wed, 3 Apr 2024 09:38:07 -0400 | Subject | Re: [PATCH 1/2] cgroup/cpuset: Make cpuset hotplug processing synchronous | From | Waiman Long <> |
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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
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