Devices or device layers can set this to control the queue selection
performed by dev_pick_tx().
This function runs under RCU protection, which allows overriding
functions to have some way of synchronizing with things like dynamic
->real_num_tx_queues adjustments.
This makes the spinlock prefetch in dev_queue_xmit() a little bit
less effective, but that's the price right now for correctness.
Signed-off-by: David S. Miller <davem@davemloft.net>
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