On Wed, 03/16 18:41, Paolo Bonzini wrote: > > > 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
It probably means BBs won't have a "home AioContext" too, in that case. > 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. If none of the bdrv_drained_begin callers is in hot path, I think we can simply call disable on each aio context that can send request to BB/BDS. Fam
