linux-kernel - Re: [PATCH] xen-blkback: Switch to closed state after releasing the backing device

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Message-ID: <20180906162932.7qge5dmrgyqbmbbj@mac.bytemobile.com>
Date:   Thu, 6 Sep 2018 18:29:32 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Valentin Vidic <Valentin.Vidic@...Net.hr>
CC:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, <xen-devel@...ts.xenproject.org>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, <drbd-user@...ts.linbit.com>
Subject: Re: [PATCH] xen-blkback: Switch to closed state after releasing the
 backing device

On Wed, Sep 05, 2018 at 06:28:01PM +0200, Valentin Vidic wrote:
> On Wed, Sep 05, 2018 at 01:35:15PM +0200, Valentin Vidic wrote:
> > > AFAICT, this will cause the backend to never switch to 'Closed' state
> > > until the toolstack sets online to 0, which is not good IMO.
> > > 
> > > If for example a frontend decides to close a device, the backend will
> > > stay in state 'Closing' until the toolstack actually removes the disk
> > > by setting online to 0.
> > > 
> > > This will prevent resetting blk connections, as blkback will refuse to
> > > switch to state XenbusStateInitWait unless it's at XenbusStateClosed
> > > (see the XenbusStateInitialising case in frontend_changed), which will
> > > never be reached with your patch.
> 
> Would it be possible to call xen_vbd_free before the state change?
> 
>         case XenbusStateClosed:
>                 xen_blkif_disconnect(be->blkif);
>                 xen_vbd_free(&be->blkif->vbd);
>                 xenbus_switch_state(dev, XenbusStateClosed);

I think that will break reconnection, since xen_vbd_create is only
called after hotplug script execution is performed (which happens only
once at device attachment), but not when DomU changes frontend
state.

If you want to perform this xen_vbd_free you will also have to move
the xen_vbd_create call AFAICT, to a place that's also called when
reconnecting a device. Note that I could be wrong, so it might be
worth a shot to try different approaches since the blkback code is
quite tangled and I might miss something.

Thanks, Roger.