On Mon, 04 Aug 2008 20:10:33 +0900
Hisashi Hifumi <hifumi.hisashi@xxxxxxxxxxxxx> wrote:
> Hi
>
> Dio write returns EIO when try_to_release_page fails because bh is
> still referenced.
> The patch
> "commit 3f31fddfa26b7594b44ff2b34f9a04ba409e0f91
> Author: Mingming Cao <cmm@xxxxxxxxxx>
> Date: Fri Jul 25 01:46:22 2008 -0700
>
> jbd: fix race between free buffer and commit transaction
> "
> was merged into 2.6.27-rc1, but I noticed that this patch is not enough
> to fix the race.
> I did fsstress test heavily to 2.6.27-rc1, and found that dio write still
> sometimes got EIO through this test.
> The patch above fixed race between freeing buffer(dio) and committing
> transaction(jbd) but I discovered that there is another race,
> freeing buffer(dio) and ext3/4_ordered_writepage.
> : background_writeout()
> ->write_cache_pages()
> ->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)
> walk_page_buffers() <-release a bh ref
>
> ext3_ordered_writepage holds bh ref and does unlock_page remaining
> taking a bh ref, so this causes the race and failure of
> try_to_release_page.
>
> Following patch fixes this race.
Please don't patch both filesystems in a single patch - they go into
the tree via different routes.
>
> Signed-off-by :Hisashi Hifumi <hifumi.hisashi@xxxxxxxxxxxxx>
"Signed-off-by: ", please.
>
> 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()?
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