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    SubjectRe: kgdb support in vanilla 2.6.2
    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" <> wrote:
    >>>On Friday 06 Feb 2004 5:46 pm, Andi Kleen wrote:
    >>>>On Fri, 6 Feb 2004 17:28:36 +0530
    >>>>"Amit S. Kale" <> wrote:
    >>>>>On Friday 06 Feb 2004 7:50 am, Andi Kleen wrote:
    >>>>>>On Thu, 5 Feb 2004 23:20:04 +0530
    >>>>>>"Amit S. Kale" <> wrote:
    >>>>>>>On Thursday 05 Feb 2004 8:41 am, Andi Kleen wrote:
    >>>>>>>>Andrew Morton <> 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
    Preemption patch:

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