Date: Thu, 15 Dec 2016 11:55:53 +0900
From: Stafford Horne <shorne@gmail.com>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Guenter Roeck <linux@roeck-us.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Build failures due to missing 'posix_timer_event'
Message-ID: <20161215025553.GG2650@lianli.shorne-pla.net>
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Hello,

On Wed, Dec 14, 2016 at 10:14:39AM -0500, Nicolas Pitre wrote:
> On Wed, 14 Dec 2016, Guenter Roeck wrote:
> 
> > avr32:allnoconfig:
> > 
> > kernel/built-in.o: In function `do_adjtimex':
> > (.text+0x1d748): undefined reference to `posix_timer_event'
> > make[1]: *** [vmlinux] Error 1
> > 
> > metag:allnoconfig:
> > 
> > kernel/built-in.o: In function `alarm_handle_timer':
> > alarmtimer.c:(.text.alarm_handle_timer+0x38): undefined reference to
> > `posix_timer_event'
> > 
> > openrisc:allnoconfig:
> > 
> > kernel/built-in.o: In function `alarm_handle_timer':
> > alarmtimer.c:(.text+0x3fca4): undefined reference to `posix_timer_event'
> > 
> > unicore32:allnoconfig:
> > 
> > kernel/built-in.o: In function `alarm_handle_timer':
> > memremap.c:(.text+0x31c1c): undefined reference to `posix_timer_event'
> > 
> > Since I don't run allnoconfig builds for all architectures, I strongly suspect
> > that more are affected.
> 
> This is a known problem. Oddly enough only those "less popular" 
> architectures are affected.
> 
> > Looking into alarmtimer.c, with seems to call posix_timer_event() 
> > unconditionally, I don't entirely understand how this can work in the 
> > first place. Maybe I am missing something, though.
> 
> The magic is in alarmtimer_init():
> 
>         if (IS_ENABLED(CONFIG_POSIX_TIMERS)) {
>                 posix_timers_register_clock(CLOCK_REALTIME_ALARM, &alarm_clock);
>                 posix_timers_register_clock(CLOCK_BOOTTIME_ALARM, &alarm_clock);
>         }
> 
> If CONFIG_POSIX_TIMERS is not enabled, the condition becomes a constant 
> zero. The posix_timers_register_clock calls are then optimized away by 
> the compiler. In turn the alarm_clock structure has no longer any 
> reference to it and therefore is also optimized away, including its 
> method functions such as alarm_timer_create() and finally 
> alarm_handle_timer().
> 
> But for some reasons, some gcc versions (mainly for "secondary" 
> architectures) fail to optimize away all that code, and a dead reference 
> to posix_timer_event() remains in the compiled code.
> 
> There is a fix already queued in the mm tree. It goes like this:
> 
>     posix-timers: give lazy compilers some help optimizing code away
>     
>     The OpenRISC compiler (so far) fails to optimize away a large portion
>     of code containing a reference to posix_timer_event in alarmtimer.c when
>     CONFIG_POSIX_TIMERS is unset. Let's give it a direct clue to let the build
>     succeed.

To be fair, this might be related to compiler version only.  I guess its
not an excuse, but when i compile with or1k-elf-gcc (version 5.3.0) I
dont get this issue. 

I guess other archs are not breaking because they are being tested is a
newer compiler?

Tested will allnoconfig

Compiler version:

  Using built-in specs.
  COLLECT_GCC=or1k-elf-gcc
  COLLECT_LTO_WRAPPER=/opt/shorne/software/or1k/libexec/gcc/or1k-elf/5.3.0/lto-wrapper
  Target: or1k-elf
  Configured with: ../or1k-gcc/configure --target=or1k-elf --prefix=/opt/shorne/software/or1k --enable-languages=c,c++ --disable-shared --disable-libssp --with-newlib
  Thread model: single
  gcc version 5.3.0 (GCC) 

-Stafford
  
>     This fixes
>     [linux-next:master 6682/7183] alarmtimer.c:undefined reference to `posix_timer_event'
>     reported by kbuild test robot.
>     
>     Signed-off-by: Nicolas Pitre <nico@linaro.org>
>     Cc: Thomas Gleixner <tglx@linutronix.de>
>     Cc: Josh Triplett <josh@joshtriplett.org>
>     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> 
> diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
> index 9b08ca391a..3921cf7fea 100644
> --- a/kernel/time/alarmtimer.c
> +++ b/kernel/time/alarmtimer.c
> @@ -516,7 +516,8 @@ static enum alarmtimer_restart alarm_handle_timer(struct alarm *alarm,
>  
>  	spin_lock_irqsave(&ptr->it_lock, flags);
>  	if ((ptr->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE) {
> -		if (posix_timer_event(ptr, 0) != 0)
> +		if (IS_ENABLED(CONFIG_POSIX_TIMERS) &&
> +		    posix_timer_event(ptr, 0) != 0)
>  			ptr->it_overrun++;
>  	}
>