Re: [PATCH 3/3] rv: Add explicit lockdep context for reactors

[Nam Cao <namcao@linutronix.de>]

Thomas Weißschuh <thomas.weissschuh@linutronix.de> writes:
> Reactors can be called from any context through tracepoints.
> When developing reactors care needs to be taken to only call APIs which
> are safe. As the tracepoints used during testing may not actually be
> called from restrictive contexts lockdep may not be helpful.
>
> Add explicit overrides to help lockdep find invalid code patterns.
>
> The usage of LD_WAIT_FREE will trigger lockdep warnings in the panic
> reactor. These are indeed valid warnings but they are out of scope for
> RV and will instead be fixed by the printk subsystem.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
...
>  void rv_react(struct rv_monitor *monitor, const char *msg, ...)
>  {
> +	static DEFINE_WAIT_OVERRIDE_MAP(rv_react_map, LD_WAIT_FREE);
>  	va_list args;
>  
>  	if (!rv_reacting_on() || !monitor->react)
> @@ -487,7 +489,9 @@ void rv_react(struct rv_monitor *monitor, const char *msg, ...)
>  
>  	va_start(args, msg);
>  
> +	lock_map_acquire_try(&rv_react_map);
>  	monitor->react(msg, args);
> +	lock_map_release(&rv_react_map);
>  
>  	va_end(args);
>  }

The reactors are invoked in tracepoints' handlers, thus they must not
trigger another tracepoint, otherwise we may be stuck in an infinite loop.
(this is why preempt_enable_notrace() exists alongside preempt_enable()).

I'm not familiar with the internal lockdep. But I think these would
trigger trace_lock_acquire() and trace_lock_release().

Nam