Merge branch 'macsec-use-rcu_work-to-fix-crypto-cleanup-in-softirq-context'

Jinliang Zheng says:

====================
macsec: use rcu_work to fix crypto cleanup in softirq context

From: Jinliang Zheng <alexjlzheng@tencent.com>

crypto_free_aead() can internally call vunmap() (e.g. via dma_free_attrs()
in hardware crypto drivers like hisi_sec2), which must not be invoked from
softirq context. Both free_rxsa() and free_txsa() are RCU callbacks that
run in softirq, causing a kernel crash on affected hardware.

This series fixes the issue by deferring the actual cleanup to a workqueue
using rcu_work, which combines the RCU grace period and workqueue dispatch
into a single primitive.

Two design decisions worth noting:

1. rcu_work instead of schedule_work() + synchronize_rcu()

   An alternative would be to call schedule_work() directly from
   macsec_rxsa_put()/macsec_txsa_put(), then call synchronize_rcu() at
   the start of the work handler to replace the grace period previously
   provided by call_rcu(). However, synchronize_rcu() blocks the worker
   thread for the duration of a full RCU grace period. Under high SA
   churn (e.g. tearing down an interface with many SAs), each SA would
   occupy a worker thread while waiting, and multiple concurrent calls
   cannot share the same grace period — leading to unnecessary latency
   and resource waste.

   rcu_work uses call_rcu_hurry() internally, which is fully asynchronous:
   the worker thread is only dispatched after the grace period has elapsed,
   and multiple concurrent queue_rcu_work() calls naturally batch under the
   same grace period via the RCU subsystem's existing coalescing mechanism.

2. Dedicated workqueue instead of system_wq

   Using a dedicated workqueue (macsec_wq) allows macsec_exit() to drain
   exactly the work items belonging to this module — by calling
   destroy_workqueue() after rcu_barrier(). If system_wq were used,
   flush_scheduled_work() would drain all pending work items across the
   entire system, creating unnecessary coupling with unrelated subsystems
   and potentially causing unexpected delays. The dedicated workqueue
   provides a clean, contained teardown path.
====================

Link: https://patch.msgid.link/20260511153102.2640368-1-alexjlzheng@tencent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>