linux-kernel - Re: 4KSTACKS + DEBUG

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Date:	Wed, 29 Aug 2007 19:55:04 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jesper Juhl <jesper.juhl@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: 4KSTACKS + DEBUG_STACKOVERFLOW harmful

On Aug 29, 2007, at 19:01:57, Eric Sandeen wrote:
> Jesper Juhl wrote:
>> A first step could be to allocate those two char arrays with  
>> kmalloc() instead of on the stack, but then I guess that dump_stack 
>> () gets called from places where we may not really want to be  
>> calling kmalloc(). I guess we could allocate the buffers earlier  
>> (like at boot time) and store pointers somewhere where dump stack  
>> can get to them later when it needs them.
>
> Yep, I thought about something like that... and I assume you'd need  
> a bit of locking around them too.

How about turning off preemption and using a per-CPU buffer?   
Alternatively you could turn off IRQs, poke a per-CPU value to clue  
in any incoming NMIs, and switch to a separate stack.  I suppose if  
you wanted it to work with all of 16 bytes of stack left on both  
thread and IRQ stacks, you could have separate per-CPU NMI stacks;  
the stack-dump would be poking a special per-CPU value and sending  
ourselves an NMI.

There are probably a half dozen other variants on ways to run  
screaming to the CPU saying "It hurts mommy!" and get a new stack in  
which we can play for a while.

Cheers,
Kyle Moffett
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