Re: [Qemu-devel] [PATCH] block: fix bdrv_flush() ordering in bdrv_close()

[Kevin Wolf <kwolf@redhat.com>]

Am 01.07.2013 um 13:29 hat Stefan Hajnoczi geschrieben:
> Since 80ccf93b we flush the block device during close.  The
> bdrv_drain_all() call should come before bdrv_flush() to ensure guest
> write requests have completed.  Otherwise we may miss pending writes
> when flushing.
> 
> However, there is still a slight change that cancelling a blockjob or
> doing bdrv_flush() might leave an I/O request running, so call
> bdrv_drain_all() again for safety as the final step.
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 6c493ad..fca55b1 100644
> --- a/block.c
> +++ b/block.c
> @@ -1358,11 +1358,12 @@ void bdrv_reopen_abort(BDRVReopenState *reopen_state)
>  
>  void bdrv_close(BlockDriverState *bs)
>  {
> -    bdrv_flush(bs);
> +    bdrv_drain_all(); /* complete guest I/O */
>      if (bs->job) {
>          block_job_cancel_sync(bs->job);
>      }
> -    bdrv_drain_all();
> +    bdrv_flush(bs);
> +    bdrv_drain_all(); /* in case blockjob or flush started I/O */
>      notifier_list_notify(&bs->close_notifiers, bs);

I think we need the bdrv_drain_all() immediately before calling
bdrv_flush(), so that block job writes are flushed as well. You can
probably move the first one there, there doesn't seem to be a reason to
drain guest requests and block job requests separately.

Kevin