From: "Amit S. Kale" <amitkale@emsyssoft.com>
Organization: EmSysSoft
To: George Anzinger <george@mvista.com>
Subject: Re: kgdb support in vanilla 2.6.2
Date: Mon, 1 Mar 2004 15:08:23 +0530
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Cc: Andi Kleen <ak@suse.de>, akpm@osdl.org, pavel@ucw.cz,
       linux-kernel@vger.kernel.org, piggy@timesys.com,
       trini@kernel.crashing.org
References: <20040204230133.GA8702@elf.ucw.cz.suse.lists.linux.kernel> <200402061914.38826.amitkale@emsyssoft.com> <403FDB37.2020704@mvista.com>
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On Saturday 28 Feb 2004 5:35 am, George Anzinger wrote:
> 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.

Having all user modifiable variables in one place is definitely a good idea.  
Need to do this sometime.
-Amit
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