linux-kernel - Re: Mysterious lockdep warning from the SCSI workqueue code

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Message-Id: <200712222356.27311.rjw@sisk.pl>
Date:	Sat, 22 Dec 2007 23:56:26 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	James Bottomley <James.Bottomley@...eleye.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Mysterious lockdep warning from the SCSI workqueue code

On Saturday, 22 of December 2007, James Bottomley wrote:
> I've no idea why this is occurring:
> 
> WARNING: at kernel/lockdep.c:700 look_up_lock_class()
> Pid: 2068, comm: scsi_wq_3 Not tainted 2.6.24-rc6 #38
>  [<c010535a>] show_trace_log_lvl+0x1a/0x30
>  [<c0105ce2>] show_trace+0x12/0x20
>  [<c010601c>] dump_stack+0x6c/0x80
>  [<c014696d>] __lock_acquire+0x46d/0x10b0
>  [<c0147628>] lock_acquire+0x78/0xa0
>  [<c0136d18>] run_workqueue+0x128/0x1d0
>  [<c013795c>] worker_thread+0x8c/0xf0
>  [<c013ab32>] kthread+0x42/0x70
>  [<c0104f47>] kernel_thread_helper+0x7/0x10
>  =======================
> 
> it looks to be happening on the initial workqueue.c:run_workqueue()
> 
> 	spin_lock_irq(&cwq->lock);
> 
> It's caused by lockdep.c:look_up_lock_class()
> 
> 	WARN_ON_ONCE(class->name != lock->name);
> 
> but I'm not entirely sure what this actually means, not fully
> understanding lockdep classes and how we acquire them in scsi.

I guess Ingo and/or Peter should have a look at this.

Thanks,
Rafael
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