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SubjectRe: [PATCH] Workqueue lockup: Circular dependency in threads
Hello,

On Thu, Aug 31, 2017 at 06:43:56PM +0530, Prateek Sood wrote:
> > 6) cpuset_mutex is acquired by task init:1 and is waiting for cpuhotplug lock.

Yeah, this is the problematic one.

> > We can reorder the sequence of locks as in the below diff to avoid this
> > deadlock. But I am looking for inputs/better solution to fix this deadlock.
> >
> > ---
> > diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> > /**
> > * update_tasks_cpumask - Update the cpumasks of tasks in the cpuset.
> > * @cs: the cpuset in which each task's cpus_allowed mask needs to be changed
> > @@ -930,7 +946,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
> > rcu_read_unlock();
> >
> > if (need_rebuild_sched_domains)
> > - rebuild_sched_domains_locked();
> > + rebuild_sched_domains_unlocked()(without taking cpuhotplug.lock)
> > }
> >
> > /**
> > @@ -1719,6 +1735,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
> > + get_online_cpus();
> > mutex_lock(&cpuset_mutex);
> > if (!is_cpuset_online(cs))
> > goto out_unlock;
> > @@ -1744,6 +1761,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
> > mutex_unlock(&cpuset_mutex);
> > + put_online_cpus();
> > kernfs_unbreak_active_protection(of->kn);
> > css_put(&cs->css);
> > flush_workqueue(cpuset_migrate_mm_wq);
> >

And the patch looks good to me. Can you please format the patch with
proper description and sob?

Thanks.

--
tejun

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