Re: [Qemu-devel] [PATCH 07/16] block: change drain to look only at one child at a time

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/03/2016 17:39, Stefan Hajnoczi wrote:
> The tree looks like this:
> 
>           [NBD export]
>              /
>             v
> [guest] temporary qcow2
>    \    /
>     v  v
>     disk
> 
> Block backend access is in square brackets.  Nodes without square
> brackets are BDS nodes.
> 
> If the guest wants to drain the disk, it's possible for new I/O requests
> to enter the disk BDS while we're recursing to disk's children because
> the NBD export socket fd is in the same AIOContext.  The socket fd is
> therefore handled during aio_poll() calls.
> 
> I'm not 100% sure that this is a problem, but I wonder if you've thought
> about this?

I hadn't, but I think this is handled by using
bdrv_drained_begin/bdrv_drained_end instead of bdrv_drain.  The NBD
export registers its callback as "external", and it is thus disabled
between bdrv_drained_begin and bdrv_drained_end.

It will indeed become more complex when BDSes won't have anymore a "home
AioContext" due to multiqueue.  I suspect that we should rethink the
strategy for enabling and disabling external callbacks.  For example we
could add callbacks to each BlockBackend that enable/disable external
callbacks, and when bdrv_drained_begin is called on a BDS, we call the
callbacks for all BlockBackends that are included in this BDS.  I'm not
sure if there's a way to go from a BDS to all the BBs above it.

Paolo