From: Jarek Poplawski <jarkao2@gmail.com>
Date: Tue, 12 Aug 2008 07:00:05 +0000

quoted text
> Of course I've to miss something, but I still don't get it: after
> synchronize_rcu() in dev_deactivate() we are sure anyone in
> dev_queue_xmit() rcu block has to see the change to noop_qdisc(),
> so it can only lose packets and not really enqueue().

The qdisc pointer traverses to the softirq handler, which can be run
in a process context (via ksoftirqd), and this pointer gets there
via the per-cpu ->output_queue.

quoted text
> IMHO the only problem is this __netif_schedule(), which could be
> done with dev_queues instead of Qdiscs with proper dereferencing
> there.  (BTW, I think we need rcu_read_lock() instead of the _bh()
> version in dev_queue_xmit() to match this with rcu_call() or
> synchronize_rcu().)

I didn't see it possible to keep scheduling the netdev_queues, as the
qdiscs can be shared with multiple queues.

Qdisc "are we running?" and other state pieces are now inside of the
Qdisc itself.  And all of the qdisc_run() and netif_schedule logic is,
as a result, Qdisc centric.

The synchronization object is the qdisc.  So we can't resample the
qdisc after scheduling it, because then the qdisc attached to the
netdev_queue can change and we'd be holding the root lock for
the wrong qdisc object.
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