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

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.

Cheers,
Stephen

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