Re: kgdb support in vanilla 2.6.2

[George Anzinger <george@mvista.com>]

Amit S. Kale wrote:
> On Friday 06 Feb 2004 6:54 pm, Andi Kleen wrote:
> 
>>On Fri, 6 Feb 2004 18:35:16 +0530
>>
>>"Amit S. Kale" <amitkale@emsyssoft.com> wrote:
>>
>>>On Friday 06 Feb 2004 5:46 pm, Andi Kleen wrote:
>>>
>>>>On Fri, 6 Feb 2004 17:28:36 +0530
>>>>
>>>>"Amit S. Kale" <amitkale@emsyssoft.com> wrote:
>>>>
>>>>>On Friday 06 Feb 2004 7:50 am, Andi Kleen wrote:
>>>>>
>>>>>>On Thu, 5 Feb 2004 23:20:04 +0530
>>>>>>
>>>>>>"Amit S. Kale" <amitkale@emsyssoft.com> wrote:
>>>>>>
>>>>>>>On Thursday 05 Feb 2004 8:41 am, Andi Kleen wrote:
>>>>>>>
>>>>>>>>Andrew Morton <akpm@osdl.org> writes:
>>>>>>>>
>>>>>>>>>need to take a look at such things and really convice
>>>>>>>>>ourselves that they're worthwhile.  Personally, I'd only be
>>>>>>>>>interested in the basic stub.
>>>>>>>>
>>>>>>>>What I found always extremly ugly in the i386 stub was that it
>>>>>>>>uses magic globals to talk to the page fault handler. For the
>>>>>>>>x86-64 version I replaced that by just using __get/__put_user
>>>>>>>>in the memory accesses, which is much cleaner. I would suggest
>>>>>>>>doing that for i386 too.
>>>>>>>
>>>>>>>May be I am missing something obvious. When debugging a page
>>>>>>>fault handler if kgdb accesses an swapped-out user page doesn't
>>>>>>>it deadlock when trying to hold mm semaphore?
>>>>>>
>>>>>>Modern i386 kernels don't grab the mm semaphore when the access is
>>>>>>
>>>>>>>= TASK_SIZE and the access came from kernel space (actually I see
>>>>>>
>>>>>>x86-64 still does, but that's a bug, will fix). You could only see
>>>>>>a deadlock when using user addresses and you already hold the mm
>>>>>>semaphore for writing (normal read lock is ok). Just don't do that.
>>>>>
>>>>>OK. It don't deadlock when kgdb accesses kernel addresses.
>>>>>
>>>>>When a user space address is accessed through kgdb, won't the kernel
>>>>>attempt to fault in the user page? We don't want that to happen
>>>>>inside kgdb.
>>>>
>>>>Yes, it will. But I don't think it's a bad thing. If the users doesn't
>>>>want that they should not follow user addresses. After all kgdb is for
>>>>people who know what they are doing.
>>>
>>>Let kgdb refuse to access any addresses below TASK_SIZE. That's better
>>>than accidentally typing something and getting lost.
>>
>>That's fine. But can you perhaps add a magic command that enables it again?
> 
> 
> Yes. This sounds good.

This could be a flag in the kgdb_info structure.  See -mm kgdb.  Does not 
require any new commands as it is just a global the user can change.


-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml