Proposal using RCU for callbacks

[Stephen Hemminger <stephen@networkplumber.org>]

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The BPF and packet capture Rx/Tx callbacks use a per-queue use counter
so that removal can wait for the datapath. In BPF that costs a locked
instruction per burst. The code would be faster and cleaner if RCU
could be used, but DPDK RCU is limited to the QSBR model, which
requires application opt-in.

The userspace RCU library (liburcu) supports other models, so I did a
brief investigation into whether something similar could be done in
DPDK. As expected, it also turned up pre-existing issues: the counters
are wrong with MT_LOCKFREE Tx, the pdump one is not safe as written,
and cryptodev callback removal never waits.

I used Claude to write it up in more detail (attached). It is all from
code reading; nothing has been reproduced or prototyped yet.
This is too big to go into 26.11 at this point, will try and have something
for 27.03

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