linux-kernel - Re: workqueue: WARN at at kernel/workqueue.c:2176

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Date:	Fri, 16 May 2014 08:16:31 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	jjherne@...ux.vnet.ibm.com, Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: workqueue: WARN at at kernel/workqueue.c:2176

On Fri, May 16, 2014 at 02:14:05PM +0200, Thomas Gleixner wrote:
> On Fri, 16 May 2014, Tejun Heo wrote:
> > On Fri, May 16, 2014 at 01:57:37PM +0200, Peter Zijlstra wrote:
> > > This of course leaves the question how the workqueue code manages to
> > > call set_cpu_allowed_ptr() on a cpu _before_ its online.
> > > 
> > > That too sounds fishy.. with the proposed patch the
> > > set_cpus_allowed_ptr() will 'gracefully' fail, but calling it in the
> > > first place is of course dubious too.
> > 
> > Right after being created, a workqueue worker invokes
> > set_cpus_allowed_ptr() to the target cpumask without checking whether
> > the cpu[s] are online or not and it's allowed to fail.  The guarantee
> > there is that the worker is already registered by that point and if a
> > CPU comes online after the registration, CPU_ONLINE notification will
> > update the cpumask accordingly, so either way the worker is guaranteed
> > to be on the right cpumask.
> 
> That's what the kthread_create_on_cpu/kthread_park/kthread_unpark
> infrastructure is for.

They aren't necessarily on one CPU.  Unbound workqueues can have
arbitrary cpumasks associated with them.

Thanks.

-- 
tejun
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