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Date: Thu, 31 Aug 2017 10:09:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: WARNING: possible circular locking dependency detected
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On Thu, Aug 31, 2017 at 09:55:57AM +0200, Thomas Gleixner wrote:
> > Arghh!!!
> > 
> > And allowing us to create events for offline CPUs (possible I think, but
> > maybe slightly tricky) won't solve that, because we're already holding
> > the hotplug_lock during PREPARE.
> 
> There are two ways to cure that:
> 
> 1) Have a pre cpus_write_lock() stage which is serialized via
>    cpus_add_remove_lock, which is the outer lock for hotplug.
> 
>    There we can sanely create stuff and fail with all consequences.

True, if you're willing to add more state to that hotplug thing I'll try
and make that perf patch that allows attaching to offline CPUs.