linux-kernel - Re: [PATCH v5 3/3] Add BUG_XX() debugging hard/soft lockup detection

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Date:	Wed, 3 Feb 2016 10:23:42 -0700
From:	Jeffrey Merkey <jeffmerkey@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	atomlin@...hat.com, cmetcalf@...hip.com, fweisbec@...il.com,
	hidehiro.kawai.ez@...achi.com, mhocko@...e.cz, tj@...nel.org,
	uobergfe@...hat.com
Subject: Re: [PATCH v5 3/3] Add BUG_XX() debugging hard/soft lockup detection

> Hmm, I am confused here.  So you are saying because we are in the nmi
> handler you can not break into the system?  The nmi handler prints some
> stuff to the screen, pokes the other cpus to print stuff to the screen and
> then returns to a normal operation.  Unless you are saying the act of
> sending NMI IPIs never completes (because a cpu is blocking IPI
> interrupts),
> so the cpu hangs in nmi context and the debugger never has a chance to
> 'break' in and see what is going on?
>
> Cheers,
> Don
>

Yes.  the nmi handlers never complete for the bug I worked on with
tglx, probably because an nmi handler is calling timekeeper.c
somewhere.  Some of these lockup bugs may be calling code from the nmi
handlers that cause the lockup condition in the first place in some
cases, so it will never reach a call to panic.  Looking over this code
it's damn hard to find a good way to do this that works across all the
arches without adding another macro to bug.h (BREAK_ON maybe), so I
just used one that's already there.  I'll go back and rethink this
some more.  It could just be as simple as calling panic from the first
detection -- that works.

Jeff