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    SubjectRe: hugetlbfs lockdep spew revisited.
    On Thu, Feb 16, 2012 at 07:08:57PM -0500, Dave Jones wrote:
    > Remember this ? https://lkml.org/lkml/2011/4/15/272
    > Josh took a stab at fixing it in e096d0c7e2e4e5893792db865dd065ac73cf1f00,
    > but it seems to still be there.

    > the existing dependency chain (in reverse order) is:

    [snip]

    ... and as bloody usual, that mentioning of readdir in the output is a
    red herring; the real problem (and yes, it *is* deadlock-prone) is not
    with getdents(2) that cannot happen on anything that could be mmaped;
    it's with hugetlbfs_read() (i.e. read(2)) that very definitely *can*.

    This is *not* a misannotation and not a false positive; this is a real,
    honest deadlock.
    Thread A:
    read() on hugetlbfs
    hugetlbfs_read() called
    i_mutex grabbed
    hugetlbfs_read_actor() called
    __copy_to_user() called
    page fault is triggered
    Thread B, sharing address space with A:
    mmap() the same file
    ->mmap_sem is grabbed on task_B->mm->mmap_sem
    hugetlbfs_file_mmap() is called
    attempt to grab ->i_mutex and block waiting for A to give it up
    Thread A:
    pagefault handled blocked on attempt to grab task_A->mm->mmap_sem,
    which happens to be the same thing as task_B->mm->mmap_sem. Block waiting
    for B to give it up.

    Deadlock.


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