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    SubjectRe: VM: Fix nasty and subtle race in shared mmap'ed page writeback
    On Wed, 3 Jan 2007 20:44:36 -0800 (PST)
    Linus Torvalds <torvalds@osdl.org> wrote:

    > Actually, I think 2.6.18 may have a subtle variation on it.
    >
    > In particular, I look back at the try_to_free_buffers() thing that I hated
    > so much, and it makes me wonder.. It used to do:
    >
    > spin_lock(&mapping->private_lock);
    > ret = drop_buffers(page, &buffers_to_free);
    > spin_unlock(&mapping->private_lock);
    > if (ret) {
    > .. crappy comment ..
    > if (test_clear_page_dirty(page))
    > task_io_account_cancelled_write(PAGE_CACHE_SIZE);
    > }
    >
    > and I think that at least on SMP, we had a race with another CPU doing the
    > "mark page dirty if it was dirty in the PTE" at the same time. Because the
    > marking dirty would come in, find no buffers (they just got dropped), and
    > then mark the page dirty (ignoring the lack of any buffers), but then the
    > above would do the "test_clear_page_dirty()" thing on it.
    >

    That bug was introduced in 2.6.19, with the dirty page tracking patches.

    2.6.18 and earlier used ->private_lock coverage in try_to_free_buffers() to
    prevent it.

    > Ie the race, I think, existed where that crappy comment was.

    The comment was complete, accurate and needed.
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