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

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Message-ID: <4C9D0A82.5020406@windriver.com>
Date:	Fri, 24 Sep 2010 15:30:58 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	a.p.zijlstra@...llo.nl, kgdb-bugreport@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: kgdb segv in the latest tip due to perf ctx changes

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@...llo.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.

View attachment "0001-Fix-null-dereference-when-using-early-kgdb.patch" of type "text/x-diff" (823 bytes)