From: Jarek Poplawski <jarkao2@gmail.com>
Date: Sat, 26 Jul 2008 15:18:38 +0200

quoted text
> I guess some additional synchronization will be added yet to prevent
> parallel freeze and especially unfreeze.

Yes, that could be a problem.  Using test_and_set_bit() can
guard the freezing sequence itself, but it won't handle
letting two threads of control freeze and unfreeze safely
without a reference count.

We want this thing to be able to be used flexbly, which means
we can't just assume that this is a short code sequence and
the unfreeze will come quickly.  That pretty much rules
out using a new lock around the operation or anything
like that.

So I guess we could replace the state bit with a reference
count.  It doesn't even need to be atomic since it is set
and tested under dev_queue->_xmit_lock

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