linux-kernel - Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb causes unkillable spinning

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Message-Id: <20071030.235600.254712083.davem@davemloft.net>
Date:	Tue, 30 Oct 2007 23:56:00 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	npiggin@...e.de
Cc:	duaneg@...da.com, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb
 causes unkillable spinning

From: Nick Piggin <npiggin@...e.de>
Date: Wed, 31 Oct 2007 07:42:21 +0100

> Sysrq+T fails to show the stack trace of a running task. Presumably this
> is to avoid a garbled stack, however it can often be useful, and besides
> there is no guarantee that the task won't start running in the middle of
> show_stack(). If there are any correctness issues, then the archietcture
> would have to take further steps to ensure the task is not running.
> 
> Signed-off-by: Nick Piggin <npiggin@...e.de>

This is useful.

Even more useful would be a show_regs() on the cpu where running tasks
are running.  If not a full show_regs() at least a program counter.

That's usually what you're trying to debug and we provide nearly no
way to handle: some task is stuck in a loop in kernel mode and you
need to know exactly where that is.

This is pretty easy to do on sparc64.  In fact I can capture remote
cpu registers even when that CPU's interrupts are disabled.  I suppose
other arches could do a NMI'ish register capture like this as well.

I have a few bug reports that I can't make more progress on because I
currently can't ask users to do something to fetch the registers on
the seemingly hung processor.  This is why I'm harping on this so
much :-)

Anyways, my core suggestion is to add a hook here so platforms can
do the remote register fetch if they want.
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