From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755466Ab0IYNzn (ORCPT ); Sat, 25 Sep 2010 09:55:43 -0400 Received: from mail-fx0-f46.google.com ([209.85.161.46]:50868 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754693Ab0IYNzm (ORCPT ); Sat, 25 Sep 2010 09:55:42 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=rCdbaH+HeZUdrFBE5C9C46pgZUUZ8yiCItyHcSKrTpOmAC9wW73LppbE7dNv0/8c17 jegAr/WN8WlZ0fnpXi774j1ecnTBNvhG1fw+RfEnEVEUfYeGydLyotPS2bHRSJs4r9fD +BCIXVxtBMTW7/nALhJ1hXMdHduQaLJMd+OwQ= Date: Sat, 25 Sep 2010 15:55:35 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: Jason Wessel , Jiri Olsa , kgdb-bugreport@lists.sourceforge.net, linux-kernel@vger.kernel.org Subject: Re: kgdb segv in the latest tip due to perf ctx changes Message-ID: <20100925135532.GA5349@nowhere> References: <20100924180417.GC1818@jolsa.brq.redhat.com> <4C9D0A82.5020406@windriver.com> <1285374560.2275.1054.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1285374560.2275.1054.camel@laptop> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Sep 25, 2010 at 02:29:20AM +0200, Peter Zijlstra wrote: > 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? Jason's patch is partially good, it just lacks one place to handle. Jiri, can you test that? diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c index d71a987..d727c58 100644 --- a/kernel/hw_breakpoint.c +++ b/kernel/hw_breakpoint.c @@ -134,7 +134,7 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp, enum bp_type_idx type) { int cpu = bp->cpu; - struct task_struct *tsk = bp->ctx->task; + struct task_struct *tsk = bp->ctx ? bp->ctx->task : NULL; if (cpu >= 0) { slots->pinned = per_cpu(nr_cpu_bp_pinned[type], cpu); @@ -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) { > > 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. Nope it's ok. tsk is used to know if we are dealing with a task/cpu bound breakpoint or a cpu wide bound one. If tsk ends up being NULL, it will think it's a cpu wide bound breakpoint, which it is in the case of kgdb breakpoints.