On Tue, Sep 30, 2014 at 08:34:06AM -0400, Rik van Riel wrote:

On 09/30/2014 07:56 AM, Arnd Bergmann wrote:

quoted

A recent change to update the stime/utime members of task_struct 
using atomic cmpxchg broke configurations on 32-bit machines with 
CONFIG_VIRT_CPU_ACCOUNTING_GEN set, because that uses 64-bit 
nanoseconds, leading to a link-time error:

kernel/built-in.o: In function `cputime_adjust': :(.text+0x25234):
undefined reference to `__bad_cmpxchg'

This reverts the change that caused the problem, I suspect the real
fix is to conditionally use cmpxchg64 instead, but I have not
checked if that will work on all architectures.

I see that kernel/sched/clock.c uses cmpxchg64 in a non
architecture, non 64 bit specific piece of code, and
nobody complained about that file not building, so I have
to assume cmpxchg64 works :)

That code is only ever used on x86 and ia64, most other archs have
managed to not mess up their clocks quite as bad.