Re: [Qemu-devel] [Qemu-block] [PATCH v2 1/2] blockdev: release the AioContext at drive_backup_prepare

[Kevin Wolf <kwolf@redhat.com>]

Am 13.09.2019 um 21:54 hat John Snow geschrieben:
> 
> 
> On 9/13/19 11:25 AM, Sergio Lopez wrote:
> > do_drive_backup() already acquires the AioContext, so release it
> > before the call.
> > 
> > Signed-off-by: Sergio Lopez <slp@redhat.com>
> > ---
> >  blockdev.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/blockdev.c b/blockdev.c
> > index fbef6845c8..3927fdab80 100644
> > --- a/blockdev.c
> > +++ b/blockdev.c
> > @@ -1783,20 +1783,16 @@ static void drive_backup_prepare(BlkActionState *common, Error **errp)
> >  
> >      aio_context = bdrv_get_aio_context(bs);
> >      aio_context_acquire(aio_context);
> > -

Are you removing this unrelated empty line intentionally?

> >      /* Paired with .clean() */
> >      bdrv_drained_begin(bs);
> 
> Do we need to make this change to blockdev_backup_prepare as well?

Actually, the whole structure feels a bit wrong. We get the bs here and
take its lock, then release the lock again and forget the reference,
only to do both things again inside do_drive_backup().

The way snapshots work is that the "normal" snapshot commands are
wrappers that turn it into a single-entry transaction. Then you have
only one code path where you can resolve the ID and take the lock just
once. So maybe backup should work like this, too?

Kevin