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    SubjectRe: 2.6.9-rc2-mm4 ps hang ?
    Hugh Dickins <hugh@veritas.com> wrote:
    >
    > lock_page inside mmap_sem a ranking bug? Please recant!

    generic_file_buffered_write() can take mmap_sem for reading while holding a
    page lock. In that rare case where the page gets unmapped even though we
    manually faulted it in.

    Now, that's lock_page->down_read versus down_read->lock_page which I
    _think_ is safe, due to down_read semantics. Even if a third thread is
    waiting for a down_write.

    Except filemap_nopage() does lock_page too, so we have

    lock_page->down_read->lock_page

    as well.

    All this does mean that down_write cannot nest either inside or outside
    lock_page.

    The bigger problem is ext3 and reiser3 transaction start/stop. It is
    equivalent to a down()/up() operation and we get the ranking for that
    inconsistent too. Both wrt lock_page and wrt, I think, down_read(mmap_sem).

    generic_file_buffered_write() does, effectively

    lock_page
    ->transaction_start
    ->fault
    ->down_read(mmap_sem)
    ->lock_page

    and over in do_mmap_pgoff() we nest transaction start inside
    down_write(mmap_sem):

    do_mmap_pgoff
    ->down_write(mmap_sem)
    ->generic_file_mmap
    ->file_accessed
    ->mark_inode_dirty
    ->transaction start

    It's all a bit of a mess. Chris Mason and I have discussed it on and off.
    I think Chris has a workload which actually does trigger a deadlock.

    Maybe dropping and retaking mmap_sem in generic_file_mmap would be a
    sufficient stopgap.
    -
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