From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1754506AbYHTHY1@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754506AbYHTHY1 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 20 Aug 2008 03:24:27 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752452AbYHTHYS
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Wed, 20 Aug 2008 03:24:18 -0400
Received: from gate.crashing.org ([63.228.1.57]:49799 "EHLO gate.crashing.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752263AbYHTHYR (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 20 Aug 2008 03:24:17 -0400
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
       Steven Rostedt <srostedt@redhat.com>,
       Scott Wood <scottwood@freescale.com>,
       Eran Liberty <liberty@extricom.com>, Alan Modra <amodra@au1.ibm.com>,
       Segher Boessenkool <segher@kernel.crashing.org>
In-Reply-To: <1219119431.8062.35.camel@pasglop>
References: <48591941.4070408@extricom.com> <48A92E15.2080709@extricom.com>
	 <48A9901B.1080900@redhat.com> <20080818154746.GA26835@Krystal>
	 <48A9AFA7.8080508@freescale.com>
	 <alpine.DEB.1.10.0808181425090.796@gandalf.stny.rr.com>
	 <1219110814.8062.2.camel@pasglop>
	 <alpine.DEB.1.10.0808182208110.13401@gandalf.stny.rr.com>
	 <1219113549.8062.13.camel@pasglop> <1219114600.8062.15.camel@pasglop>
	 <alpine.DEB.1.10.0808182304300.13401@gandalf.stny.rr.com>
	 <1219119431.8062.35.camel@pasglop>
Content-Type: text/plain
Date: Wed, 20 Aug 2008 17:18:25 +1000
Message-Id: <1219216705.21386.46.camel@pasglop>
Mime-Version: 1.0
X-Mailer: Evolution 2.22.3.1 
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Found the problem (or at least -a- problem), it's a gcc bug.

Well, first I must say the code generated by -pg is just plain
horrible :-)

Appart from that, look at the exit of, for example, __d_lookup, as
generated by gcc when ftrace is enabled:

c00c0498:       38 60 00 00     li      r3,0
c00c049c:       81 61 00 00     lwz     r11,0(r1)
c00c04a0:       80 0b 00 04     lwz     r0,4(r11)
c00c04a4:       7d 61 5b 78     mr      r1,r11
c00c04a8:       bb 0b ff e0     lmw     r24,-32(r11)
c00c04ac:       7c 08 03 a6     mtlr    r0
c00c04b0:       4e 80 00 20     blr

As you can see, it restores r1 -before- it pops r24..r31 off
the stack ! I let you imagine what happens if an interrupt happens
just in between those two instructions (mr and lmw). We don't do
redzones on our ABI, so basically, the registers end up corrupted
by the interrupt.

Cheers,
Ben.