linux-kernel - Re: mod_delayed_work() explosion due to 874bbfe6

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Date:	Wed, 3 Feb 2016 11:32:29 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, tangchen@...fujitsu.com,
	rafael@...nel.org
Subject: Re: mod_delayed_work()  explosion due to  874bbfe6

On Tue, Feb 02, 2016 at 03:44:24PM +0100, Mike Galbraith wrote:
> Scenario: CPU168 calls mod_delayed_work(), is taken offline before the
> timer expires.  Due to 874bbfe6, dwork->cpu is the now offline CPU168
> vs the previous WORK_CPU_UNBOUND, timer fires on CPU131, it tries to
> __queue_work() with cpu == the now offline CPU168, gets to...
> 
>         } else
>                 pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));
> 
> ... and goes boom.
> 
> <snippet>
> crash> p numa_node | grep 168
>   [168]: ffff8c03fdf0e328
> crash> rd ffff8c03fdf0e328
> ffff8c03fdf0e328:  00000000ffffffff                    ........
> 
> Thus, pwq becomes 000000000. Then, as the result of reference to
> pwq->pool, NULL reference occurs at (*PANIC).
> </snippet>
> 
> What if anything is supposed to prevent this?

(cc'ing 

So, the root problem here is cpu <-> node mapping flipping across cpu
on/offlining which is unnecessary and causes other issues too.  It's
being worked on for quite a while now.

 http://lkml.kernel.org/g/1453702100-2597-1-git-send-email-tangchen@cn.fujitsu.com

I'll create a bandaid patch for now so that wq falls back to dfl_wq on
negative node.

Thanks.

-- 
tejun