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Date: Tue, 22 Mar 2016 12:59:58 +0100
From: Cornelia Huck <cornelia.huck@de.ibm.com>
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Subject: Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>, tu bo <tubo@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>

On Tue, 22 Mar 2016 10:46:58 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 22/03/2016 10:07, Cornelia Huck wrote:
> > So far, we had the best results with my refactoring + the mutex/bh
> > change. Two problems:
> > 
> > - We don't really understand yet why my refactoring helps, but passing
> > the assign value is a good canditate (and it's agreed that this fixes a
> > bug, I think.)
> > - There's some problem with the bh, if I understood Stefan correctly.
> 
> They can be fixed with just an extra object_ref/object_unref.
> 
> I didn't understand that Tu Bo also needed the BH fix, and with that
> information it makes sense.  Passing the assign value ensures that
> ioeventfd remains always assigned.  With the CPU threads out of the
> picture, the BH becomes enough to make everything thread-safe.

Yes, this makes sense.

Might we still have a hole somewhere in dataplane teardown? Probably
not, from reading the code, even if it runs in cpu thread context.