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: <51C18540.5060200@asianux.com>
Date:	Wed, 19 Jun 2013 18:17:36 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Tejun Heo <tj@...nel.org>
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 06/19/2013 04:41 PM, Tejun Heo wrote:
> On Wed, Jun 19, 2013 at 12:03:38PM +0800, Chen Gang wrote:
>> > 
>> > Since "WARN_ON(worker->task)", we can not assume that 'worker->task'
>> > will be NULL before set 'current' to it.
>> > 
>> > So need let 'worker' lock protected too, just like it already lock
>> > protected all time in main looping.
> That synchronization is the kthread_worker user's responsibility.  The
> locking around worker->task = NULL is to prevent the worker task being
> destroyed while insert_kthread_work() is trying to wake it up.  It has
> nothing to do with the user trying to attach multiple tasks to the
> same kthread_worker.  Plus, putting locking around WARN_ON() is
> pointless.  It doesn't really fix anything.  It just makes WARN_ON()
> trigger *slightly* more reliably.

Hmm... can 'worker->task' has chance to be not NULL before set 'current'
to it ?

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

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


Thanks.
-- 
Chen Gang

Asianux Corporation
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