Re: [PATCH 1/3] rv/reactors: fix lockdep "Invalid wait context" in rv_react()

[Nam Cao <namcao@linutronix.de>]

wen.yang@linux.dev writes:
> The DEFINE_WAIT_OVERRIDE_MAP() macro creates a lockdep map with
> wait_type_inner = LD_WAIT_CONFIG, which inherits the outer context's
> wait type.  When rv_react() is called from a LD_WAIT_FREE context
> (e.g., a KUnit test with busy-wait), and the reactor callback triggers
> a timer interrupt during the busy-loop,

I am confused by the last sentence. How can reactor callback triggers a
timer interrupt?

Do you mean a timer interrupt happens in the middle of the reactor
callback? And this only happens sporadically, right?

> the interrupt exit path attempts
> to schedule (preempt_schedule_irq -> __schedule -> rq->__lock), which is
> LD_WAIT_SPIN.  Lockdep then reports:
>
>     [ BUG: Invalid wait context ]
>     context-{5:5}
>     1 lock held by kunit_try_catch/209:
>      #0: rv_react_map-wait-type-override at rv_react+0x9d/0xf0
>
> The wait_type_override map allowed the outer LD_WAIT_FREE to propagate
> inward, but scheduling from an interrupt is LD_WAIT_SPIN, violating the
> constraint.
>
> Fix by explicitly setting wait_type_inner = LD_WAIT_SPIN, which is the
> tightest constraint rv_react() callbacks must satisfy: they may not
> sleep (LD_WAIT_SLEEP) or use mutexes, but can use spinlocks and be
> interrupted. This matches the documented LD_WAIT_FREE constraint.

These concepts are new to me. Let me do some studying before reviewing.

Nam