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Re: [Qemu-block] [Qemu-devel] [PATCH 07/16] block: change drain to look

From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 07/16] block: change drain to look only at one child at a time
Date: Thu, 17 Mar 2016 08:57:14 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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