A lot of drivers follow the same scheme to stop / start queues
without introducing locks between xmit and NAPI tx completions.
I'm guessing they all copy'n'paste each other's code.
The original code dates back all the way to e1000 and Linux 2.6.19.

Smaller drivers shy away from the scheme and introduce a lock
which may cause deadlocks in netpoll.

Provide macros which encapsulate the necessary logic.

The macros do not prevent false wake ups, the extra barrier
required to close that race is not worth it. See discussion in:
https://lore.kernel.org/all/c39312a2-4537-14b4-270c-9fe1fbb91e89@gmail.com/



Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>