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Date: Wed, 21 Aug 2013 14:47:12 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130821124712.GA11975@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 2/4] nbd: call drive_put_ref() only if
 dinfo exists
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To: Fam Zheng <famz@redhat.com>
Cc: kwolf@redhat.com, pbonzini@redhat.com, imain@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com

On Mon, Jul 29, 2013 at 12:25:30PM +0800, Fam Zheng wrote:
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  blockdev-nbd.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/blockdev-nbd.c b/blockdev-nbd.c
> index 95f10c8..c75df19 100644
> --- a/blockdev-nbd.c
> +++ b/blockdev-nbd.c
> @@ -72,7 +72,10 @@ static void nbd_close_notifier(Notifier *n, void *data)
>  static void nbd_server_put_ref(NBDExport *exp)
>  {
>      BlockDriverState *bs = nbd_export_get_blockdev(exp);
> -    drive_put_ref(drive_get_by_blockdev(bs));
> +    DriveInfo *dinfo = drive_get_by_blockdev(bs);
> +    if (dinfo) {
> +        drive_put_ref(dinfo);
> +    }

This doesn't make sense to me.  If we hold a reference, we must release
it.

Once your refcount series is applied, NBD should be able to hold just
the BDS refcount.  Then we don't care about DriveInfo at all anymore
besides looking up the BDS when the user creates an export.

Stefan