Re: On trace_*_rcuidle functions in modules

[Paul E. McKenney]

On Tue, Apr 14, 2020 at 07:20:01PM -0700, John Stultz wrote:
> Hey folks,
>   So recently I was looking at converting some drivers to be loadable
> modules instead of built-in only, and one of my patches just landed in
> -next and started getting build error reports.
> 
> It ends up, recently in the merge window, the driver I was converting
> to module switched a trace_*() function to trace_*_rcuidle() to fix a
> bug.  Now when building as a module, if tracing is configured on, it
> can't seem to find the trace_*_rcuidle() symbol.
> 
> This is because, as you are aware, we don't declare trace_*_rcuidle
> functions in modules - and haven't for quite some time:
>   https://lore.kernel.org/lkml/20120905062306.GA14756@leaf/
> 
> I wanted to better understand the background rationale for that patch,
> to understand if not exporting the rcu_idle_exit and rcu_idle_enter,
> calls was because they weren't used or if it was a more intentional
> decision to avoid allowing modules to use them.
> 
> Would it be reasonable to revisit that patch? Or is there some
> recommended alternative solution?

I will defer to Steven and Josh on the rationale.  (Cowardly of me,
I know!)

What I do is to maintain a wrapper for tracepoints within a built-in
portion of RCU, export the wrapper, and invoke the wrapper from the
rcutorture module.  Maybe you can do something similar?

But why would a module be invoked from the idle loop?  Is the module
supplying an idle driver or some such?

							Thanx, Paul