Re: Possible deadlock related to CPU hotplug and kernfs

On Mon, Sep 07, 2015 at 11:33:21PM +0200, Rafael J. Wysocki wrote:
> On Monday, September 07, 2015 11:11:19 AM Jiang Liu wrote:
> Peter, Ingo, some help from lockdep expert is needed.
> 
> We have a splat that almost certainly is a false positive (the original report
> is here http://marc.info/?l=linux-kernel&m4109156901959&w=4) and no ideas
> how to make it go away.  Can you please have a look and advise?

I can't even find the relevant code :/

From that email I get kernfs_fop_write() which calls
kernfs_get_active(), but that does _NOT_ call cpu_up(), so that
callchain is shite.

The actual lockdep splat is also not really helpful, and is spraying
names over: acpi, device, sysfs and kernfs (do we really need that many
layeres of obfuscation for a simple file?)

So, please, start by explaining the thing proper such that simple people
like me know what to look for.