linux-kernel - Re: [Patch 0/2] sysfs: fix s

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Date:	Fri, 05 Feb 2010 11:43:31 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Miles Lane <miles.lane@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Larry Finger <Larry.Finger@...inger.net>,
	akpm@...ux-foundation.org, Alan Stern <stern@...land.harvard.edu>
Subject: Re: [Patch 0/2] sysfs: fix s_active lockdep warning

Peter Zijlstra wrote:
> On Fri, 2010-01-29 at 12:30 -0800, Eric W. Biederman wrote:
> 
>> We get false positives when the code of a sysfs attribute
>> synchronously removes other sysfs attributes.  In general that is not
>> safe due to hotplug etc, but there are specific instances of static
>> sysfs entries like the pm_core where it appears to be safe.
>>
>> I am not familiar with the device core lockdep issues.  Are they similar?
> 
> The device tree had the problem that we could basically hold a device
> lock and an unspecified number of parent locks (iirc this was due to
> device probing, where we hold the bus lock while probing/adding child
> device, recursively). 
> 
> If we place each dev->lock into the same class (which would naively
> happen), then this would lead to recursive lock warnings. The proposed
> solution for this is to create MAX_LOCK_DEPTH classes and assign them to
> the dev->lock depending on the depth in the device tree (Alan said that
> MAX_LOCK_DEPTH is sufficient for all practical cases).
> 
> static struct lock_class_key dev_tree_classes[MAX_LOCK_DEPTH];
> 
> device_add() or thereabouts would have something like:
> 
> #ifdef CONFIG_PROVE_LOCKING
> 	BUG_ON(dev->depth >= MAX_LOCK_DEPTH);
> 	lockdep_set_class(dev->lock, &dev_tree_classes[dev->depth]);
> #endif
> 
> 

Nice explanation!

I see, we should set the class of the lock when after we holding it...
I will update my patch.

Thank you!
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