linux-kernel - Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot)

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Date:   Tue, 11 Oct 2016 00:22:49 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        torvalds@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        akpm@...ux-foundation.org, kernel-team@...com,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org
Subject: Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue
 available early during boot)



On 10/10/16 23:53, Tejun Heo wrote:
> On Mon, Oct 10, 2016 at 10:17:16PM +1100, Balbir Singh wrote:
>> rest_init()
>> {
>> 	...
>> 	kernel_thread(kernel_init, NULL, CLONE_FS);
>> 	numa_default_policy();
>> 	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
>> 	rcu_read_lock();
>> 	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>> 	...
>>
>> }
>>
>> create_worker() needs kthreadd, it wakes up kthreadd in kthread_create_on_node,
>> workqueue_init() is called from kernel_init() , but kthreadd is created after
>> the call to kernel_init(), so its touch and go
> 
> But the first thing kernel_init_freeable() does is
> wait_for_completion(&kthreadd_done).
> 

Yes, Of course, looking at the stack trace again, it was not the wake_up itself,
but the absence of cfs_rq of p->se that caused the issue. Will try and chase it
down. Quick look shows cgroup_init() has occurred before workqueue_init(), so
ideally p->se.cfs_rq should be allocated.

Sorry for the noise,

Balbir