Re: kgdb support in vanilla 2.6.2

[Andi Kleen]

On Fri, 6 Feb 2004 17:28:36 +0530
"Amit S. Kale" <amitkale@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> wrote:
> > > On Thursday 05 Feb 2004 8:41 am, Andi Kleen wrote:
> > > > Andrew Morton <akpm@xxxxxxxx> 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.

-Andi
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