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

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

[Valdis Klētnieks]

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commit 9088b449814f788d24f35a5840b6b2c2a23cd32a
Author: Paul E. McKenney <paulmck@kernel.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?

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

[Randy Dunlap]

On 5/28/20 9:16 PM, Valdis Klētnieks wrote:
> commit 9088b449814f788d24f35a5840b6b2c2a23cd32a
> Author: Paul E. McKenney <paulmck@kernel.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?
> 

Paul has already responded: (unfortunately)

"So I am restricting to 64BIT for the time being.  Yeah, I know, lazy of
me.  ;-)"

https://lore.kernel.org/lkml/20200528180855.GP2869@paulmck-ThinkPad-P72/

-- 
~Randy

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

[Valdis Klētnieks]

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On Thu, 28 May 2020 21:48:18 -0700, Randy Dunlap said:

> > ERROR: modpost: "__aeabi_uldivmod" [kernel/rcu/refperf.ko] undefined!

Gaah.  And the reason I didn't spot Paul's post while grepping my linux-kernel
mailbox is because *that* thread had a different undefined reference:

> > > > > > m68k-linux-ld: kernel/rcu/refperf.o: in function `main_func':
> > > > > > >> refperf.c:(.text+0x762): undefined reference to `__umoddi3'

> Paul has already responded: (unfortunately)
>
> "So I am restricting to 64BIT for the time being.  Yeah, I know, lazy of
> me.  ;-)"

It's the sort of issue that's well into "as long as it gets mostly fixed before
it hits Linus's tree" territory.   I've seen lots of far worse work-arounds in
the years since the 2.5.47 kernel. :)






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

[Paul E. McKenney]

On Fri, May 29, 2020 at 01:24:39AM -0400, Valdis Klētnieks wrote:
> On Thu, 28 May 2020 21:48:18 -0700, Randy Dunlap said:
> 
> > > ERROR: modpost: "__aeabi_uldivmod" [kernel/rcu/refperf.ko] undefined!
> 
> Gaah.  And the reason I didn't spot Paul's post while grepping my linux-kernel
> mailbox is because *that* thread had a different undefined reference:
> 
> > > > > > > m68k-linux-ld: kernel/rcu/refperf.o: in function `main_func':
> > > > > > > >> refperf.c:(.text+0x762): undefined reference to `__umoddi3'
> 
> > Paul has already responded: (unfortunately)
> >
> > "So I am restricting to 64BIT for the time being.  Yeah, I know, lazy of
> > me.  ;-)"
> 
> It's the sort of issue that's well into "as long as it gets mostly fixed before
> it hits Linus's tree" territory.   I've seen lots of far worse work-arounds in
> the years since the 2.5.47 kernel. :)

Fair enough!

Also as noted on the other thread, in this case, doing this 64-bit
division the hard way shouldn't be a problem:  The performance test is
finished and nothing else is happening.  So I have to wonder whether it
would be possible to detect this based on some sort of link-time checking,
presumably in conjunction with -O0 to avoid confusing the whitelist with
compiler optimizations.

That would allow use of C-language "/" and "%" while still allowing
gratuitous uses to be questioned.

Hey, I can dream, can't I?  ;-)

							Thanx, Paul

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

[Paul E. McKenney]

On Fri, May 29, 2020 at 12:16:31AM -0400, Valdis Klētnieks wrote:
> commit 9088b449814f788d24f35a5840b6b2c2a23cd32a
> Author: Paul E. McKenney <paulmck@kernel.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