linux-kernel - Re: Possible deadlock related to CPU hotplug and kernfs

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Date:	Tue, 8 Sep 2015 12:40:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>, mingo@...nel.org,
	"Rafael J. Wysocki" <rafael@...nel.org>, Tejun Heo <tj@...nel.org>,
	linux hotplug mailing <linux-hotplug@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: 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&m=144109156901959&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.
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