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is queued to the hardware. Because multiple concurrent paths can execute ath_start(), multiple concurrent paths can push frames into the software/hardware TX queue and since preemption/interrupting can occur, there's the possibility that a gap in time will occur between allocating the sequence number and queuing it to the hardware. Because of this, it's possible that a thread will have allocated a sequence number and then be preempted by another thread doing the same. If the second thread sneaks the frame into the BAW, the (earlier) sequence number of the first frame will be now outside the BAW and will result in the frame being constantly re-added to the tail of the queue. There it will live until the sequence numbers cycle around again. This also creates a hole in the RX BAW tracking which can also cause issues. This patch delays the sequence number allocation to occur only just before the frame is going to be added to the BAW. I've been wanting to do this anyway as part of a general code tidyup but I've not gotten around to it. This fixes the PR. However, it still makes it quite difficult to try and ensure in-order queuing and dequeuing of frames. Since multiple copies of ath_start() can be run at the same time (eg one TXing process thread, one TX completion task/one RX task) the driver may end up having frames dequeued and pushed into the hardware slightly/occasionally out of order. And, to make matters more annoying, net80211 may have the same behaviour - in the non-aggregation case, the TX code allocates sequence numbers before it's thrown to the driver. I'll open another PR to investigate this and potentially introduce some kind of final-pass TX serialisation before frames are thrown to the hardware. It's also very likely worthwhile adding some debugging code into ath(4) and net80211 to catch when/if this does occur. PR: kern/166190
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