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: <20130619084124.GF30681@mtj.dyndns.org>
Date:	Wed, 19 Jun 2013 01:41:24 -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 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.

Thanks.

-- 
tejun
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