Re: [PATCH] blockdev: fix drive-backup transaction endless drained section

[Eric Blake <eblake@redhat.com>]

On Tue, Jun 08, 2021 at 08:18:52PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> drive_backup_prepare() does bdrv_drained_begin() in hope that
> bdrv_drained_end() will be called in drive_backup_clean(). Still we
> need to set state->bs for this to work. That's done too late: a lot of
> failure paths in drive_backup_prepare() miss setting state->bs. Fix
> that.
> 
> Fixes: 2288ccfac96281c316db942d10e3f921c1373064
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/399
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  blockdev.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/blockdev.c b/blockdev.c
> index f08192deda..094c085962 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1714,6 +1714,7 @@ static void drive_backup_prepare(BlkActionState *common, Error **errp)
>      aio_context = bdrv_get_aio_context(bs);
>      aio_context_acquire(aio_context);
>  
> +    state->bs = bs;
>      /* Paired with .clean() */
>      bdrv_drained_begin(bs);

Commit 2288ccfac9 had these two lines in the opposite order, but that
doesn't matter, the important part is that we do indeed need to set
state->bs regardless of any later failure detections, to get .clean to
do the matching drained_end.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org