Re: [PATCH] xen/blkback: fix disconnect while I/Os in flight

[Roger Pau Monné]

On Fri, Apr 21, 2017 at 10:51:00AM +0200, Juergen Gross wrote:
> Today disconnecting xen-blkback is broken in case there are still
> I/Os in flight: xen_blkif_disconnect() will bail out early without
> releasing all resources in the hope it will be called again when
> the last request has terminated. This, however, won't happen as
> xen_blkif_free() won't be called on termination of the last running
> request: xen_blkif_put() won't decrement the blkif refcnt to 0 as
> xen_blkif_disconnect() didn't finish before thus some xen_blkif_put()
> calls in xen_blkif_disconnect() didn't happen.
> 
> To solve this deadlock xen_blkif_disconnect() and
> xen_blkif_alloc_rings() shouldn't use xen_blkif_put() and
> xen_blkif_get() but use some other way to do their accounting of
> resources.
> 
> This at once fixes another error in xen_blkif_disconnect(): when it
> returned early with -EBUSY for another ring than 0 it would call
> xen_blkif_put() again for already handled rings on a subsequent call.
> This will lead to inconsistencies in the refcnt handling.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Tested-by: Steven Haigh <netwiz@xxxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

TBH, it seems like all this accounting in blkback could be improved quite a
lot. Right now this is just a complete caos.

Thanks, Roger.