Re: [Patch 0/2] sysfs: fix s_active lockdep warning

[Cong Wang]

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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