From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755469Ab0IYA3b (ORCPT ); Fri, 24 Sep 2010 20:29:31 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:53208 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753772Ab0IYA3a convert rfc822-to-8bit (ORCPT ); Fri, 24 Sep 2010 20:29:30 -0400 Subject: Re: kgdb segv in the latest tip due to perf ctx changes From: Peter Zijlstra To: Jason Wessel Cc: Jiri Olsa , kgdb-bugreport@lists.sourceforge.net, linux-kernel@vger.kernel.org, Frederic Weisbecker In-Reply-To: <4C9D0A82.5020406@windriver.com> References: <20100924180417.GC1818@jolsa.brq.redhat.com> <4C9D0A82.5020406@windriver.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Sat, 25 Sep 2010 02:29:20 +0200 Message-ID: <1285374560.2275.1054.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2010-09-24 at 15:30 -0500, Jason Wessel wrote: > Jiri, > > Can you try this simple patch which is attached? > > > > On 09/24/2010 01:04 PM, Jiri Olsa wrote: > > while starting kgdb early debug on latest tip tree, > > I got SIGSEGV inside kernel in following location: > > > > > [clip] > > I found out it's due to foolowing commit, that's causing the init code > > to be called without the ctx field being defined... > > > > commit c3f00c70276d8ae82578c8b773e2db657f69a478 > > Author: Peter Zijlstra > > Date: Wed Aug 18 14:37:15 2010 +0200 > > > > > > > > I took a look at the tip core, and the ctx parameter is no longer passed > into the perf_event_alloc() from perf_event_create_kernel_counter() kgdb > no longer gets it filled in for free. > > The reality is that kgdb never had a true context or a way to mark the > hw breakpoint as a kernel only context for the hw breakpoint > reservations. The patch is only a short term fix perhaps until on of > the perf guys explains the right way to use it. :-) Argh, yes, its using the ctx rather early.. we cannot have a context before we've initialized the event, and here it looks like hw_breakpoint wants to use the context to initialize the event, chick, egg, etc.. Frederic, anything we can do about that? > differences between files attachment > (0001-Fix-null-dereference-when-using-early-kgdb.patch) > From 17f3febd001a26aee9a75c61152b60b7e0ae1ea9 Mon Sep 17 00:00:00 2001 > From: Jason Wessel > Date: Fri, 24 Sep 2010 15:21:11 -0500 > Subject: [PATCH] Fix null dereference when using early kgdb > > Signed-off-by: Jason Wessel > --- > kernel/hw_breakpoint.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c > index 3b714e8..3c7ccdf 100644 > --- a/kernel/hw_breakpoint.c > +++ b/kernel/hw_breakpoint.c > @@ -213,7 +213,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, > int weight) > { > int cpu = bp->cpu; > - struct task_struct *tsk = bp->ctx->task; > + struct task_struct *tsk = bp->ctx ? bp->ctx->task : NULL; > > /* Pinned counter cpu profiling */ > if (!tsk) { That'll probably screw over some accounting, not sure what tsk is used for there.