linux-kernel - Re: [PATCH 01/31] Add hard/soft lockup debugger entry points

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Date:	Thu, 28 Jan 2016 13:08:01 -0700
From:	Jeffrey Merkey <jeffmerkey@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	dzickus@...hat.com, uobergfe@...hat.com, atomlin@...hat.com,
	mhocko@...e.cz, fweisbec@...il.com, tj@...nel.org,
	hidehiro.kawai.ez@...achi.com, cmetcalf@...hip.com
Subject: Re: [PATCH 01/31] Add hard/soft lockup debugger entry points

On 1/28/16, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 28 Jan 2016, Jeffrey Merkey wrote:
>
>> This patch series adds an export which can be set by system debuggers to
>> direct the hard lockup and soft lockup detector to trigger a breakpoint
>> exception and enter a debugger if one is active.  It is assumed that if
>> someone sets this variable, then an breakpoint handler of some sort will
>> be actively loaded or registered via the notify die handler chain.
>>
>> This addition is extremely useful for debugging hard and soft lockups
>> real time and quickly from a console debugger.
>
> Why do we need an extra breakpoint instruction in the code to enter a
> debugger? Don't have debuggers mechanisms to install breakpoints?
>
> I'm probably missing something obvious here.
>
> Thanks,
>
> 	tglx
>
>
>

It's a pain in the butt to grep around through assembly language in a
function in watchdog.c that has everything declared static with no
symbols.  It's a lot easier just to insert an INT3 in the section of
code that has the mouse caught in the trap (inside a function that
triggers the hard lockup) -- after all -- that's what the instruction
is for.

Jeff