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

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Message-ID: <20251014094206-80eb5d6c-e4dd-4704-a40a-e2d0461c2185@linutronix.de>
Date: Tue, 14 Oct 2025 11:46:16 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Nam Cao <namcao@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>, 
	Gabriele Monaco <gmonaco@...hat.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] rv: Add explicit lockdep context for reactors

On Tue, Oct 14, 2025 at 09:38:09AM +0200, Nam Cao wrote:
> Thomas Weißschuh <thomas.weissschuh@...utronix.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@...utronix.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()).

Sounds reasonable. However today not even the printk reactor satisfies this
rule as it transitively calls trace_console().

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

Indeed. Right now no monitor attaches to those tracepoints. We could
prevent monitors from attaching to certain "well-known" tracepoints.
But then we still need to manually track which those are, which is ugly.
Or we move the invocation of the reactor to a workqueue/task_work.


Thomas