linux-kernel - Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference counting

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Date:	Tue, 16 Feb 2016 10:14:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:	Intel graphics driver community testing & development 
	<intel-gfx@...ts.freedesktop.org>,
	Linux kernel development <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference
 counting

On Tue, Feb 16, 2016 at 10:49:36AM +0200, Joonas Lahtinen wrote:
> I originally thought of implementing this more similar to what you
> specify, but then I came across a discussion in the mailing list where
> it was NAKed adding more members to task_struct;
> 
> http://comments.gmane.org/gmane.linux.kernel/970273
> 
> Adding proper recursion (the way my initial implementation was going)
> got ugly without modifying task_struct because get_online_cpus() is a
> speed critical code path.

Yeah, just don't let Linus hear you say that. get_online_cpus() is _not_
considered performance critical.

> So I'm all for fixing the current code in a different way if that will
> then be merged.

So I'm not sure why you're poking at this horror show to begin with.
ISTR you mentioning a lockdep splat for SKL, but failed to provide
detail.

Making the hotplug lock _more_ special to fix that is just wrong. Fix
the retarded locking that lead to it.