Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN_FLUSH

[Tao Ma <tao.ma@oracle.com>]

Hi Tejun,
	Just a small qs about this patch.

On 09/03/2010 06:29 PM, Tejun Heo wrote:
> loop implements FLUSH using fsync but was incorrectly setting its
> ordered mode to DRAIN.  Change it to DRAIN_FLUSH.  In practice, this
> doesn't change anything as loop doesn't make use of the block layer
> ordered implementation.
According to Documentation/block/barrier.txt,
QUEUE_ORDERED_DRAIN
         Requests are ordered by draining the request queue and cache
         flushing isn't needed.

         Sequence: drain => barrier

QUEUE_ORDERED_DRAIN_FLUSH
         Requests are ordered by draining the request queue and both
         pre-barrier and post-barrier cache flushings are needed.

         Sequence: drain => preflush => barrier => postflush

And for loop device, it call fsync in barrier request. See 
do_bio_filebacked in drivers/block/loop.c
bool barrier = !!(bio->bi_rw & REQ_HARDBARRIER);
                 struct file *file = lo->lo_backing_file;

                 if (barrier) {
                         if (unlikely(!file->f_op->fsync)) {
                                 ret = -EOPNOTSUPP;
                                 goto out;
                         }

                         ret = vfs_fsync(file, 0);
                         if (unlikely(ret)) {
                                 ret = -EIO;
                                 goto out;
                         }
                 }

So actually the sync is done in barrier and no extra flush is needed?
Or Do I misread the rules in barrier.txt?

Regards,
Tao

>
> Signed-off-by: Tejun Heo<tj@kernel.org>
> ---
>   drivers/block/loop.c |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index f3c636d..c3a4a2e 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -832,7 +832,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>   	lo->lo_queue->unplug_fn = loop_unplug;
>
>   	if (!(lo_flags&  LO_FLAGS_READ_ONLY)&&  file->f_op->fsync)
> -		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_DRAIN);
> +		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_DRAIN_FLUSH);
>
>   	set_capacity(lo->lo_disk, size);
>   	bd_set_size(bdev, size<<  9);