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Date: Thu, 18 Apr 2013 10:16:27 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH v4 15/15] slirp: use lock to protect
 the slirp_instances
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: mdroth <mdroth@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>, Liu Ping Fan <qemulist@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, qemu-devel@nongnu.org


> grep'ing for slirp_instances points to more spots that work with that
> list (QTAILQ_FOREACH, QTAILQ_EMPTY, ...). So the same question here:
> What are the usage rules? When do I _not_ need it when touching the list
> of instances, and why?
> 
> Well, I started reading at the top, but there are more lock-adding
> patches in this series. And the more locks we have, the higher the
> probability of ABBA gets. Therefore, please document from the beginning
> the lock order rules that shall prevent it (which may also be "never
> take other locks while holding this one" or "never hold other locks when
> taking this one").

Yeah, the only sane ordering rules should be "hold nothing or just
the BQL when taking this one".  Everything else needs a very good
justification...

Paolo