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Re: [Qemu-devel] [PATCH 07/16] block: change drain to look only at one c
Re: [Qemu-devel] [PATCH 07/16] block: change drain to look only at one child at a time
Wed, 16 Mar 2016 18:41:32 +0100
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On 16/03/2016 17:39, Stefan Hajnoczi wrote:
> The tree looks like this:
> [NBD export]
> [guest] temporary qcow2
> \ /
> v v
> 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.