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    SubjectRe: VM in 2.4.7-pre hurts...

    On Sun, 8 Jul 2001, Rik van Riel wrote:
    >
    > If __wait_on_buffer and ___wait_on_page get stuck, this could
    > mean a page doesn't get unlocked. When this is happening, we
    > may well be running into a dozens of pages which aren't getting
    > properly unlocked on IO completion.

    Absolutely. But that, in turn, should cause just others getting stuck, not
    running, no?

    Anyway, having looked at the buffer case, I htink I found a potentially
    nasty bug: "unlock_buffer()" with a buffer cout of zero.

    Why is this nasty? unlock_buffer() does:

    extern inline void unlock_buffer(struct buffer_head *bh)
    {
    clear_bit(BH_Lock, &bh->b_state);
    smp_mb__after_clear_bit();
    if (waitqueue_active(&bh->b_wait))
    wake_up(&bh->b_wait);
    }

    but by doing the "clear_bit()", it also potentially free's the buffer, so
    an interrupt coming in (or another CPU) can end up doing a kfree() on the
    bh.

    At which point the "waitqueue_active()" and the wakeup call are operating
    on random memory.

    This does not explain __wait_on_buffer(), but it's a bug none-the-less.

    Anybody can find anything else fishy with buffer handling?

    Linus

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