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    SubjectRe: [Ext2-devel] [RFC] probably bug in current ext3/jbd

    On Sun, 2003-05-18 at 18:21, Alex Tomas wrote:

    > ext3/jbd use b_committed_data buffer in order to prevent
    > allocation of blocks which were freed in non-committed
    > transaction. I think there is bug in this code. look,

    You just found a rather subtle bug which was fixed around 2 or 3 years
    ago. :-)

    > some thread commit thread
    > ----------------------------------------------------------

    > start_journal()
    > get_undo_access(#1):
    > 1) wait for #1 to be
    > in t_forget_list

    get_undo_access is a declaration of intention to modify the buffer.
    When that happens, it calls do_get_write_access() with the force_copy
    flag set. That means that it _always_ creates a new frozen_data copy of
    the buffer the first time we get undo access to a bitmap buffer within
    any given transaction. That basically means that for bitmaps,
    frozen_data always holds the version of the buffer as of the end of the
    previously completed transaction.

    > for_each_bh_in_forget_list() {
    > if (jh->b_committed_data) {
    > kfree(jh->b_committed_data);
    > jh->b_committed_data = NULL;
    > }

    Ah, but the *immediately* following lines are:

    if (jh->b_frozen_data) {
    jh->b_committed_data = jh->b_frozen_data;
    jh->b_frozen_data = NULL;

    so the frozen data that was preserved at get_undo_access() time has now
    committed to disk and gets rotated into the b_committed_data version.
    This is exactly how we get the new version of the committed data when
    the old transaction commits.


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