> -----Original Message-----
> From: John Snow <js...@redhat.com>
> Sent: 20 September 2019 22:11
> To: Paul Durrant <paul.durr...@citrix.com>; xen-de...@lists.xenproject.org;
> qemu-de...@nongnu.org;
> qemu-block@nongnu.org
> Cc: Kevin Wolf <kw...@redhat.com>; Stefano Stabellini
> <sstabell...@kernel.org>; Max Reitz
> <mre...@redhat.com>; Anthony Perard <anthony.per...@citrix.com>; Mark Syms
> <mark.s...@citrix.com>
> Subject: Re: [Qemu-block] [PATCH] xen-block: treat XenbusStateUnknown the
> same as XenbusStateClosed
>
>
>
> On 9/18/19 7:57 AM, Paul Durrant wrote:
> > When a frontend gracefully disconnects from an offline backend, it will
> > set its own state to XenbusStateClosed. The code in xen-block.c correctly
> > deals with this and sets the backend into XenbusStateClosed. Unfortunately
> > it is possible for toolstack to actually delete the frontend area
> > before the state key has been read, leading to an apparent frontend state
> > of XenbusStateUnknown. This prevents the backend state from transitioning
> > to XenbusStateClosed and hence leaves it limbo.
> >
>
> Does the 0 come from a read into de-allocated memory?
No, it comes from the fact that the xenstore state key is not present.
Conventionally a missing state key means the state is reported as
XenbusStateUnknown.
Paul
>
> --js
>
> > This patch simply treats a frontend state of XenbusStateUnknown the same
> > as XenbusStateClosed, which will unblock the backend in these circumstances.
> >
> > Reported-by: Mark Syms <mark.s...@citrix.com>
> > Signed-off-by: Paul Durrant <paul.durr...@citrix.com>
> > ---
> > Cc: Stefano Stabellini <sstabell...@kernel.org>
> > Cc: Anthony Perard <anthony.per...@citrix.com>
> > Cc: Kevin Wolf <kw...@redhat.com>
> > Cc: Max Reitz <mre...@redhat.com>
> > ---
> > hw/block/xen-block.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
> > index f77343db60..879fc310a4 100644
> > --- a/hw/block/xen-block.c
> > +++ b/hw/block/xen-block.c
> > @@ -313,6 +313,7 @@ static void xen_block_frontend_changed(XenDevice
> > *xendev,
> > break;
> >
> > case XenbusStateClosed:
> > + case XenbusStateUnknown:
> > xen_block_disconnect(xendev, &local_err);
> > if (local_err) {
> > error_propagate(errp, local_err);
> >
