Re: Build failures due to missing 'posix_timer_event'

From: Nicolas Pitre
Date: Wed Dec 14 2016 - 10:14:46 EST

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():

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

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

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@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

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)
+ posix_timer_event(ptr, 0) != 0)