在 2008-08-05二的 11:36 +0900，Hisashi Hifumi写道：
> >>
> >> 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
>
Okay, I see the race window, DIO could come in before
walk_page_buffers() release the bh reference. So far I don't see a nicer
way to sync between background writeout with DIO path yet...
Mingming
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