linux-kernel - Re: driver/base/dd.c lockdep warning

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Message-ID: <20090902185315.GU28804@bolzano.suse.de>
Date:	Wed, 2 Sep 2009 20:53:16 +0200
From:	Jan Blunck <jblunck@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Kay Sievers <kay.sievers@...e.de>, gregkh@...e.de,
	kasievers@...e.de, USB list <linux-usb@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: driver/base/dd.c lockdep warning

On Wed, Sep 02, Peter Zijlstra wrote:

> 
> So the proposal I currently have to solve this is to allocate 48 lock
> classes:
> 
> struct lock_class_key device_tree_depth[MAX_LOCK_DEPTH];
> 
> and when creating a new device node, set the lock class corresponding
> the depth in the tree:
> 
>   mutex_lock_init(&device->lock);
>   BUG_ON(device->depth >= MAX_LOCK_DEPTH); // surely we're not that deep
>   lockdep_set_class(&device->lock, device_tree_depth + device->depth);
>   ...
>   mutex_lock(&device->lock); /* already have parent locked */
>   device_attach(device, parent);
> 
> and take multiple child locks using:
> 
>   mutex_lock_nest_lock(&device->lock, &device->parent->lock);
> 
> Which, I think should work for most cases out there.
> 
> Alan had some funny corner cases, but I think he wasn't sure whether
> those would indeed show up in reality.

JFYI, I tried to get away with just parent, child and normal class but
the problem is that device_add() is called recursive via driver probing
code. So either we change that (and iterate down the device tree) or we need
to implement what you proposed.

Cheers,
Jan
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