Re: kgdb segv in the latest tip due to perf ctx changes

[Jason Wessel <jason.wessel@windriver.com>]

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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 <a.p.zijlstra@chello.nl>
> 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. :-)

Thanks,
Jason.


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>From 17f3febd001a26aee9a75c61152b60b7e0ae1ea9 Mon Sep 17 00:00:00 2001
From: Jason Wessel <jason.wessel@windriver.com>
Date: Fri, 24 Sep 2010 15:21:11 -0500
Subject: [PATCH] Fix null dereference when using early kgdb

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
---
 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) {
-- 
1.6.3.3