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 S1753571Ab3C0Ohh (ORCPT <rfc822;w@1wt.eu>);
	Wed, 27 Mar 2013 10:37:37 -0400
Received: from e39.co.us.ibm.com ([32.97.110.160]:59783 "EHLO
	e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753477Ab3C0Ohg (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 27 Mar 2013 10:37:36 -0400
Date: Wed, 27 Mar 2013 07:37:15 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Namhyung Kim <namhyung.kim@lge.com>, Jiri Olsa <jolsa@redhat.com>
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code:
 asm/8267
Message-ID: <20130327143715.GY4379@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20130324115556.GA4866@pd.tnic>
 <20130324155924.GB4866@pd.tnic>
 <20130326183452.GC27518@pd.tnic>
 <20130327131532.GL30540@8bytes.org>
 <1364393852.5053.74.camel@laptop>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1364393852.5053.74.camel@laptop>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-TM-AS-MML: No
X-Content-Scanned: Fidelis XPS MAILER
x-cbid: 13032714-3620-0000-0000-000001CAFEF5
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Mar 27, 2013 at 03:17:32PM +0100, Peter Zijlstra wrote:
> On Wed, 2013-03-27 at 14:15 +0100, Joerg Roedel wrote:
> > What makes me wonder here is that the code is preemptible in an
> > rcu_read_locked section. As far as I know preemption needs to be
> > disabled while holding the rcu_read_lock().
> 
> Nah, a long long time ago some -rt people complained to paulmck that
> keeping preemption disabled over all this RCU stuff was killing
> latencies. Paul liked the challenge and came up with some mind twisting
> stuff to make it work.

What can I say?  I was young and foolish.  And I still am pretty
foolish.  ;-)

But yes, you are not required to disable preemption across
rcu_read_lock(), and rcu_read_lock() is not guaranteed to disable
preemption.  So if you need preemption to be disabled, do it explicitly
with preempt_disable(), local_irq_save(), rcu_read_lock_sched(),
or whatever, because rcu_read_lock() isn't always going to disable
preemption.

> If you're into that kind of pain, look at CONFIG_*_PREEMPT_RCU :-)

Or just set CONFIG_PREEMPT=y, which will set CONFIG_TREE_PREEMPT_RCU=y
on CONFIG_SMP=y builds and will set CONFIG_TINY_PREEMPT_RCU=y otherwise.
(But please note that CONFIG_TINY_PREEMPT_RCU is going away, after which
CONFIG_PREEMPT=y will always set CONFIG_TREE_PREEMPT_RCU=y.)

							Thanx, Paul

> But yeah, you need to have that stuff enabled before you can hit this
> particular snag.
>