[PATCH net 0/2] netpoll: fix PREEMPT_RT deferred transmit locking

[Karl Mehltretter <kmehltretter@gmail.com>]

netpoll's deferred transmit path can acquire two sleeping locks with hard
interrupts disabled on PREEMPT_RT. One is the sk_buff_head lock used by
the sender and worker. The other is the device transmit lock used by the
worker. They produce independent atomic-sleep reports.

Patch 1 gives the deferred skb queue a dedicated raw lock.

Patch 2 makes the worker try the device transmit lock and defer the skb
again when the lock is busy. It uses the queue helper added by patch 1.

Testing:

- Arm64 and x86_64 PREEMPT_RT W=1 builds passed.
- An unpatched Pi 400 emitted the queue-lock report five times during a
  one-hour run. Ten boots with the complete fix passed, including one
  3,606-second run. Every trigger and post-trigger marker arrived, Wi-Fi
  remained usable, and neither atomic-sleep warning occurred.
- An arm64 PREEMPT_RT QEMU control reproduced both warnings under forced
  transmit backpressure. Three boots with the complete fix passed.
- An x86_64 QEMU control reproduced the queue-lock warning on PREEMPT_RT.
  Three fixed RT boots passed. The non-RT control and treatment each
  passed three boots.

Karl Mehltretter (2):
  netpoll: use a raw lock for the deferred transmit queue
  netpoll: avoid blocking on the transmit lock in queue_process

 include/linux/netpoll.h |  1 +
 net/core/netpoll.c      | 83 +++++++++++++++++++++++++++++++++++++----
 2 files changed, 77 insertions(+), 7 deletions(-)


base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.53.0