Re: [Xen-devel] [PATCH 1/2] xen-bus: Fix backend state transition on device reset

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> Sent: 21 August 2019 10:20
> To: qemu-devel@xxxxxxxxxx
> Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>; qemu-stable@xxxxxxxxxx; 
> Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: [PATCH 1/2] xen-bus: Fix backend state transition on device reset
> 
> When a frontend want to reset its state and the backend one, it start
> with setting "Closing", then wait for the backend (QEMU) to do the same.
> 
> But when QEMU is setting "Closing" to its state, it trigger an event
> (xenstore watch) that re-execute xen_device_backend_changed() and set
> the backend state to "Closed". QEMU should wait for the frontend to
> set "Closed" before doing the same.
> 
> Before setting "Closed" to the backend_state, we are also going to
> check if the frontend was responsible for the transition to "Closing".
> 
> Fixes: b6af8926fb858c4f1426e5acb2cfc1f0580ec98a
> Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> ---
> Cc: qemu-stable@xxxxxxxxxx
> ---
>  hw/xen/xen-bus.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> index e40500242d..982eca4533 100644
> --- a/hw/xen/xen-bus.c
> +++ b/hw/xen/xen-bus.c
> @@ -540,9 +540,11 @@ static void xen_device_backend_changed(void *opaque)
>      /*
>       * If the toolstack (or unplug request callback) has set the backend
>       * state to Closing, but there is no active frontend (i.e. the
> -     * state is not Connected) then set the backend state to Closed.
> +     * state is not Connected or Closing) then set the backend state
> +     * to Closed.
>       */
>      if (xendev->backend_state == XenbusStateClosing &&
> +        xendev->frontend_state != XenbusStateClosing &&
>          xendev->frontend_state != XenbusStateConnected) {
>          xen_device_backend_set_state(xendev, XenbusStateClosed);

Actually, I wonder whether it is better to 'whitelist' here? AFAIK the only 
valid frontend states whether the backend should set itself 'closed' are 
'closed' (i.e. the frontend is finished) and 'initialising' (the frontend was 
never there).

  Paul

>      }
> --
> Anthony PERARD


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