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Date: Tue, 29 Aug 2017 21:49:48 +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 Tue, Aug 29, 2017 at 07:40:44PM +0200, Thomas Gleixner wrote:

> One solution I'm looking into right now is to reverse the lock order and
> actually make the hotplug code do:
> 
> 	 watchdog_lock();
> 	 cpu_write_lock();
> 
> 	 ....
> 	 cpu_write_unlock();
> 	 watchdog_unlock();
> 	 
> and get rid of cpu_read_(un)lock() in the sysctl interface completely. I
> know it's ugly, but we have other locks we take in the hotplug path as
> well.

This is to serialize the sysctl against hotplug? I'm not immediately
seeing why watchdog_lock needs to be the outer most lock, is that
because of vfs locks or something?

> That solves that part of the issue, but it does not solve the
> release_ds_buffers() problem. Though with the watchdog_lock() mechanism, it
> allows me to do:
> 
>        ->park() := watchdog_disable()
>           perf_event_disable(percpuevt);
> 	  cleanup_event = percpuevt;
> 	  percpuevt = NULL;
> and then
> 
>        watchdog_unlock()
>           if (cleanup_event) {
> 	  	perf_event_release_ebent(cleanup_event);
> 		cleanup_event = NULL;
> 	  }
> 	  mutex_unlock(&watchdog_mutex);
> 
> That should do the trick nicely for both user space functions and the cpu
> hotplug machinery.
> 
> Though it's quite a rewrite of that mess, which is particularly non trivial
> because that extra non perf implementation in arch/powerpc which has its
> own NMI watchdog thingy wants its calls preserved. But AFAICT so far it
> should just work. Famous last words....
> 
> Thoughts?

So I have a patch _somewhere_ that preserves the event<->cpu relation
across hotplug and disable/enable would be sufficient. If you want I can
try and dig that out and make it work again.

That would avoid having to do the destroy/create cycle of the watchdog
events.