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

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Date:	Thu, 20 Jun 2013 09:02:49 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chen Gang <gang.chen@...anux.com>
cc:	Tejun Heo <tj@...nel.org>, 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 Thu, 20 Jun 2013, Chen Gang wrote:

> On 06/19/2013 11:52 PM, Tejun Heo wrote:
> > 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.
> > 
> 
> If we detect the bugs, and still want to use WARN_ON() to report warning
> and continue running, we need be sure of keeping the related things no
> touch (at least not lead to worse).
> 
> If we can not be sure of keeping the related things no touch:
>   if it is a kernel bug, better use BUG_ON() instead of,
>   if it is a user mode bug, better to return failure with error code and
> print related information.

Wrong. BUG_ON() is only for cases where the kernel CANNOT continue at
all. WARN_ON() prints the very same information, but allows to
continue.

> BUG_ON() will stop current working flow and report kernel bug in details.

There is no reason to crash the machine completely. The kernel can
continue and the WARN_ON reports the bug with the same details.

Thanks,

	tglx
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