block: fix bdrv_flush() ordering in bdrv_close()

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(-)

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

On Tue, Jul 02, 2013 at 11:38:33AM +0200, Kevin Wolf wrote:
> 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.

Good idea, will fix in v2.

Stefan