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    Subject[PATCH]: Fix SMP-reordering race in mark_buffer_dirty
    Hi

    It looks like someone overoptimized mark_buffer_dirty().

    mark_buffer_dirty() is
    void mark_buffer_dirty(struct buffer_head *bh)
    {
    WARN_ON_ONCE(!buffer_uptodate(bh));
    if (!buffer_dirty(bh) && !test_set_buffer_dirty(bh))
    __set_page_dirty(bh->b_page, page_mapping(bh->b_page), 0);
    }

    That buffer_dirty() test is not atomic, it may be reordered with whatever
    else.

    So suppose this race

    CPU1:

    write to buffer data
    call mark_buffer_dirty()
    test for !buffer_dirty(bh)

    --- there is no synchronizing operation, so inside CPU it may get
    reordered to:

    test for !buffer_dirty(bh)
    write to buffer data

    CPU2:
    clear_buffer_dirty(bh);
    submit_bh(WRITE, bh);

    The resulting operations may end up in this order:
    CPU1: test for !buffer_dirty(bh) --- sees that the bit is set
    CPU2: clear_buffer_dirty(bh);
    CPU2: submit_bh(WRITE, bh);
    CPU1: write to buffer data

    So we have a clean buffer with modified data and this modification is
    going to be lost.

    Mikulas


    Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>

    --- linux-2.6.25-rc8/fs/buffer.c_ 2008-04-02 21:08:36.000000000 +0200
    +++ linux-2.6.25-rc8/fs/buffer.c 2008-04-02 21:10:25.000000000 +0200
    @@ -1180,6 +1180,12 @@
    */
    void mark_buffer_dirty(struct buffer_head *bh)
    {
    + /*
    + * Make sure that the test for buffer_dirty(bh) is not reordered with
    + * previous modifications to the buffer data.
    + * -- mikulas
    + */
    + smp_mb();
    WARN_ON_ONCE(!buffer_uptodate(bh));
    if (!buffer_dirty(bh) && !test_set_buffer_dirty(bh))
    __set_page_dirty(bh->b_page, page_mapping(bh->b_page), 0);

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      Last update: 2008-04-02 21:23    [W:0.025 / U:32.016 seconds]
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