Re: [PATCH, RFC] ext4: Replace hackishext4_mb_poll_new_transaction with commit callback

From: Theodore Tso
Date: Sun Oct 19 2008 - 21:13:44 EST

On Sun, Oct 19, 2008 at 04:49:28PM -0600, Andreas Dilger wrote:
> The problem with the mechanism you've implemented is that it isn't
> possible to add any other kind of callback to the journal. There
> is only a single callback function, and the entries in the "t_private_list"
> are all assumed to be "ext4_free_data" structures so even if other
> users wanted to add callbacks they would only be handled by the
> release_blocks_on_commit() function.
> Is there any reason not to make this more generic and have the callback
> function pointer embedded in the "ext4_free_data" struct in some way
> that other callbacks could be registered? This would still avoid the
> need to allocate for each of these operations, but would make the
> callback mechanism more generic and useful.

Another possibility would be to simply re-point
sbi->s_journal->j_commit_callback() to a function like this:

static void new_commit_callback_func(journal_t *journal, transaction_t *txn)
do_new_callback_stuff(journal, txn)
release_blocks_on_commit(journal, txn)

Or Luster could initialize ext4, and then intercept the callback,
stash it away in a pointer, and then do this:

static void new_commit_callback_func(journal_t *journal, transaction_t *txn)
do_new_callback_stuff(journal, txn)
(*orig_value_of_j_commit_callback)(journal, txn)

If you want to be even *more* general, we could hang a struct
list_head off of struct ext4_sb_info which contains the linked list of
functions to be called, one of which would be release_blocks_on_commit(),
and then change sbi->s_journal->j_commit_callback to point to a function
which iterates over the functions in the list_head

Do you have specific use in mind for Lustre? Could one of the above
schemes work for you?

- Ted
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