linux-kernel - Re: next-20200528 - build error in kernel/rcu/refperf.c

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Message-ID: <20200529135556.GY2869@paulmck-ThinkPad-P72>
Date:   Fri, 29 May 2020 06:55:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: next-20200528 - build error in kernel/rcu/refperf.c

On Fri, May 29, 2020 at 12:16:31AM -0400, Valdis Klētnieks wrote:
> commit 9088b449814f788d24f35a5840b6b2c2a23cd32a
> Author: Paul E. McKenney <paulmck@...nel.org>
> Date:   Mon May 25 17:22:24 2020 -0700
> 
>     refperf: Provide module parameter to specify number of experiments
> 
> changes this line of code (line 389)
> 
> -               reader_tasks[exp].result_avg = 1000 * process_durations(exp) / ((exp + 1) * loops);
> +               result_avg[exp] = 1000 * process_durations(nreaders) / (nreaders * loops);
> 
> On a 32-bit ARM make allmodconfig with gcc 8.3, this results in:
> 
> ERROR: modpost: "__aeabi_uldivmod" [kernel/rcu/refperf.ko] undefined!
> make[1]: *** [scripts/Makefile.modpost:103: __modpost] Error 1
> 
> I admit not understanding why the original line of code worked and the new one doesn't.
> Maybe gcc is smarter/dumber about the ranges of 'exp' and 'nreaders' than we thought?

I was surprised by that as well, but yesterday I took the lazy way out
by making this module depend on 64BIT.  (0day reported a similar issue
on m68k.)  So this issue should no longer show up.  ;-)

							Thanx, Paul