>>
>> diff -Nrup linux-2.6.27-rc1.org/fs/jbd/transaction.c
>linux-2.6.27-rc1/fs/jbd/transaction.c
>> --- linux-2.6.27-rc1.org/fs/jbd/transaction.c 2008-07-29
>19:28:47.000000000 +0900
>> +++ linux-2.6.27-rc1/fs/jbd/transaction.c 2008-07-29 20:40:12.000000000 +0900
>> @@ -1764,6 +1764,12 @@ int journal_try_to_free_buffers(journal_
>> */
>> if (ret == 0 && (gfp_mask & __GFP_WAIT) && (gfp_mask & __GFP_FS)) {
>> journal_wait_for_transaction_sync_data(journal);
>> +
>> + bh = head;
>> + do {
>> + while (atomic_read(&bh->b_count))
>> + schedule();
>> + } while ((bh = bh->b_this_page) != head);
>> ret = try_to_free_buffers(page);
>> }
>
>The loop is problematic. If the scheduler decides to keep running this
>task then we have a busy loop. If this task has realtime policy then
>it might even lock up the kernel.
>
>Perhaps we can use wait_on_page_writeback()?
>
We cannot use wait_on_page_writeback() to wait for releasing bh ref because
in ext3_ordered_writepage() bh ref is grabbed and released through walk_page_buffers
so between both walk_page_buffers, it remains taking a bh ref even if end_page_writeback
is performed.
->ext3_ordered_writepage()
walk_page_buffers() <- take a bh ref
block_write_full_page() <- unlock_page
: <- end_page_writeback
: <- race! (dio write->try_to_release_page fails): ---> remains taking a bh ref
walk_page_buffers() <-release a bh ref
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