From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754651AbcBPJPk (ORCPT <rfc822;w@1wt.eu>);
	Tue, 16 Feb 2016 04:15:40 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:36296 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754437AbcBPJOs (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 16 Feb 2016 04:14:48 -0500
Date: Tue, 16 Feb 2016 10:14:40 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Intel graphics driver community testing & development 
	<intel-gfx@lists.freedesktop.org>,
        Linux kernel development <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        David Hildenbrand <dahi@linux.vnet.ibm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference
 counting
Message-ID: <20160216091440.GT6357@twins.programming.kicks-ass.net>
References: <1455539803-13913-1-git-send-email-joonas.lahtinen@linux.intel.com>
 <20160215141755.GG6357@twins.programming.kicks-ass.net>
 <20160215170618.GL6375@twins.programming.kicks-ass.net>
 <1455612576.4977.11.camel@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <1455612576.4977.11.camel@linux.intel.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.