Re: [Qemu-devel] [PATCH 3/7] nbd: use BDS refcount

[Fam Zheng]

On Tue, 07/02 12:16, Paolo Bonzini wrote:
> Il 02/07/2013 07:59, Fam Zheng ha scritto:
> > Previously, nbd call drive_get_ref on the drive of bs. A BDS doesn't
> > always have associated dinfo, it's more proper to use bdrv_get_ref().
> 
> This has the important side effect of marking the exported disk as
> "in_use" (to use the terms before the series).  Right now you can serve
> a disk and, at the same time, stream it or mirror it or create a live
> snapshot of it.
> 
> Do we really want to block anything for a device being served?  Perhaps
> truncation, but maybe not even that.  The NBD server is meant to be as
> unobtrusive as possible (in some sense NBD accesses are the same as
> guest accesses).
> 
> Paolo
> 

OK, it is better to work like that. But I don't quite understand why was
there drive_get_ref() on the device (w/o the series), as there's already
a close notifier? And it just drive_put_ref() when bs is closed?

> > Signed-off-by: Fam Zheng <address@hidden>
> > ---
> >  blockdev-nbd.c | 9 +--------
> >  nbd.c          | 5 +++++
> >  2 files changed, 6 insertions(+), 8 deletions(-)
> > 
> > diff --git a/blockdev-nbd.c b/blockdev-nbd.c
> > index 95f10c8..d8bcd6f 100644
> > --- a/blockdev-nbd.c
> > +++ b/blockdev-nbd.c
> > @@ -69,12 +69,6 @@ static void nbd_close_notifier(Notifier *n, void *data)
> >      g_free(cn);
> >  }
> >  
> > -static void nbd_server_put_ref(NBDExport *exp)
> > -{
> > -    BlockDriverState *bs = nbd_export_get_blockdev(exp);
> > -    drive_put_ref(drive_get_by_blockdev(bs));
> > -}
> > -
> >  void qmp_nbd_server_add(const char *device, bool has_writable, bool 
> > writable,
> >                          Error **errp)
> >  {
> > @@ -106,10 +100,9 @@ void qmp_nbd_server_add(const char *device, bool 
> > has_writable, bool writable,
> >      }
> >  
> >      exp = nbd_export_new(bs, 0, -1, writable ? 0 : NBD_FLAG_READ_ONLY,
> > -                         nbd_server_put_ref);
> > +                         NULL);
> >  
> >      nbd_export_set_name(exp, device);
> > -    drive_get_ref(drive_get_by_blockdev(bs));
> >  
> >      n = g_malloc0(sizeof(NBDCloseNotifier));
> >      n->n.notify = nbd_close_notifier;
> > diff --git a/nbd.c b/nbd.c
> > index 2606403..f28b9fb 100644
> > --- a/nbd.c
> > +++ b/nbd.c
> > @@ -881,6 +881,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t 
> > dev_offset,
> >      exp->nbdflags = nbdflags;
> >      exp->size = size == -1 ? bdrv_getlength(bs) : size;
> >      exp->close = close;
> > +    bdrv_get_ref(bs);
> >      return exp;
> >  }
> >  
> > @@ -927,6 +928,10 @@ void nbd_export_close(NBDExport *exp)
> >      }
> >      nbd_export_set_name(exp, NULL);
> >      nbd_export_put(exp);
> > +    if (exp->bs) {
> > +        bdrv_put_ref(exp->bs);
> > +        exp->bs = NULL;
> > +    }
> >  }
> >  
> >  void nbd_export_get(NBDExport *exp)
> > 
> 

-- 
Fam