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

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Message-ID: <20140516120822.GA13288@mtj.dyndns.org>
Date:	Fri, 16 May 2014 08:08:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: workqueue: WARN at at kernel/workqueue.c:2176

Hello, Peter.

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.

Thanks.

-- 
tejun
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