linux-kernel - Re: [PATCH] kernel/kthread.c: need spin_lock_irq() for 'worker' before main looping, since it can "WARN

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Message-ID: <20130619155218.GA14881@htj.dyndns.org>
Date:	Wed, 19 Jun 2013 08:52:18 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>, laijs@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/kthread.c: need spin_lock_irq() for 'worker'
 before main looping,  since it can "WARN_ON(worker->task)".

On Wed, Jun 19, 2013 at 06:17:36PM +0800, Chen Gang wrote:
> Hmm... can 'worker->task' has chance to be not NULL before set 'current'
> to it ?

Yes, if the caller screws up and try to attach more than one workers
to the kthread_worker, which has some possibility of happening as
kthread_worker allows both attaching and detaching a worker.

> why do we use WARN_ON(worker->task) ?

To detect bugs on the caller side.

> I guess it still has chance to let "worker->task != NULL", or it should
> be BUG_ON(worker->task) instead of.

What difference does that make?

-- 
tejun
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