[PATCH v5 0/5] rv/reactors: fix lockdep warning and add tests

[wen.yang@linux.dev]

From: Wen Yang <wen.yang@linux.dev>

We occasionally hit a lockdep "Invalid wait context" warning in
production when a reactor callback is preempted by a timer interrupt.
On interrupt exit the scheduler takes rq->__lock (LD_WAIT_SPIN) while
rv_react() still holds its wait-type-override map, which declared
LD_WAIT_FREE.  On any kernel where the task context has preemption
enabled (not just CONFIG_PREEMPT_RT) this triggers a spurious lockdep
report:

    [ BUG: Invalid wait context ]
    1 lock held by kunit_try_catch/209:
     #0: (rv_react_map-wait-type-override){+.+.}-{1:1}
    kunit_try_catch/209 is trying to lock:
    ffff8a743ed3e8a0 (&rq->__lock){-...}-{2:2}

This series fixes the wait type (patch 1), makes reactor registration
failures propagate (patch 2), adds support for module-based
reactors (patch 3), and adds KUnit/kselftest coverage (patch 4-5).

Changes in v5:
- Patch 1: the locking rules no longer go to
  Documentation/trace/rv/monitor_synthesis.rst, which is not about
  reactors (Gabriele); they are documented in code comments.
  Functional code unchanged.
- Patch 2: unchanged
- Patch 3: redesigned. v4 only exported the registration helpers,
  which is harmless while all in-tree reactors are built-in, but
  would let a loudable reactor module be unloaded while a monitor
  still refereances its react() callback. Per the suggestion in
  commit 3d3800b4f7 ("rv: Remove rv_reactor's reference counter").
  A new selftest (patch 5) exercises it.
- Patch 4: The test callback is kept busy for two timer ticks so
  the test also passes with HZ < 200.
- Patch 5: a selftest module that registers a reactor exercises
  the module pinni g end to end: rmmod must fail while a monitor
  is attached. 

v4: https://lore.kernel.org/lkml/cover.1787854397.git.wen.yang@linux.dev/
v3: https://lore.kernel.org/lkml/cover.1786294920.git.wen.yang@linux.dev/
v2: https://lore.kernel.org/lkml/cover.1785695669.git.wen.yang@linux.dev/
v1: https://lore.kernel.org/lkml/cover.1781541556.git.wen.yang@linux.dev/

Wen Yang (5):
  rv/reactors: use LD_WAIT_SPIN as the reactor lockdep wait type
  rv/reactors: propagate rv_register_reactor() error from reactor init
  rv/reactors: export rv_register_reactor() and rv_unregister_reactor()
  rv/reactors: add KUnit tests for reactor registration and dispatch
  selftests/verification: Test loadable module-based reactor

 include/linux/rv.h                            |   3 +
 kernel/trace/rv/Kconfig                       |  12 ++
 kernel/trace/rv/Makefile                      |   1 +
 kernel/trace/rv/reactor_panic.c               |   3 +-
 kernel/trace/rv/reactor_printk.c              |   3 +-
 kernel/trace/rv/rv.c                          |   5 +
 kernel/trace/rv/rv_reactors.c                 |  48 ++++++--
 kernel/trace/rv/rv_reactors_kunit.c           | 110 ++++++++++++++++++
 tools/testing/selftests/verification/Makefile |   1 +
 tools/testing/selftests/verification/config   |   2 +
 .../test.d/rv_reactor_loadable.tc             |  46 ++++++++
 .../verification/test_modules/Makefile        |  16 +++
 .../test_modules/rv_test_reactor.c            |  37 ++++++
 13 files changed, 276 insertions(+), 11 deletions(-)
 create mode 100644 kernel/trace/rv/rv_reactors_kunit.c
 create mode 100644 tools/testing/selftests/verification/test.d/rv_reactor_loadable.tc
 create mode 100644 tools/testing/selftests/verification/test_modules/Makefile
 create mode 100644 tools/testing/selftests/verification/test_modules/rv_test_reactor.c

base-commit: 785095112f4198de49760552374f364043c8dbdf
-- 
2.25.1