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    SubjectRe: [patch 0/9] buffered write deadlock fix
    On Tue, Jan 30, 2007 at 03:21:19PM -0800, Andrew Morton wrote:
    > On Tue, 30 Jan 2007 12:55:58 -0800
    > Andrew Morton <akpm@osdl.org> wrote:
    >
    > > y'know, four or five years back I fixed this bug by doing
    > >
    > > current->locked_page = page;
    > >
    > > in the write() code, and then teaching the pagefault code to avoid locking
    > > the same page. Patch below.
    > >
    > > But then evil mean Hugh pointed out that the patch is still vulnerable to
    > > ab/ba deadlocking so I dropped it.
    >
    > And he was right, of course. Task A holds file a's i_mutex and takes a
    > fault against file b's page. Task B holds file b's i_mutex and takes a
    > fault against file a's page. Drat.
    >
    > I wonder if there's a sane way of preventing that.

    If you want to go down the path of carrying state around in task_struct,
    you can take the mmap_sem and set a flag, then get_user_pages the source
    page and lock both source and destination in ascending order, then your
    page fault handler checks the flag and skips mmap_sem, and the rest of
    your fault path checks both the page locks you're holding.

    At which point you arrive at a horrible mess :)

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