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SubjectRe: kgdb support in vanilla 2.6.2
Date
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.

-Amit


>
> > George has coded cfi directives i386 too. He can use them to backtrace
> > past irqs stack.
>
> Problem is that he did it without binutils support. I don't think that's a
> good idea because it makes the code basically unmaintainable for normal
> souls (it's like writing assembly code directly in hex)
>
> -Andi

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