net: dev: Always serialize on Qdisc::busylock in __dev_xmit_skb() on PREEMPT_RT.
The root-lock is dropped before dev_hard_start_xmit() is invoked and after setting the __QDISC___STATE_RUNNING bit. If the Qdisc owner is preempted by another sender/task with a higher priority then this new sender won't be able to submit packets to the NIC directly instead they will be enqueued into the Qdisc. The NIC will remain idle until the Qdisc owner is scheduled again and finishes the job. By serializing every task on the ->busylock then the task will be preempted by a sender only after the Qdisc has no owner. Always serialize on the busylock on PREEMPT_RT. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: David S. Miller <davem@davemloft.net>
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