For files that include asm/processor.h but not asm/system.h:

arch/arm/mach-msm/include/mach/uncompress.h: In function 'putc':
arch/arm/mach-msm/include/mach/uncompress.h:48:3: error: implicit declaration of function 'smp_mb' [-Werror=implicit-function-declaration]

In this case, smp_mb() is from the cpu_relax() call in the msm putc().

It likely went uncaught when the uncompress.h change went in since the
defconfig didn't enable that code path, but later changes (e76f4750:
ARM: debug: arrange Kconfig options more logically) resulted in the
option being on for msm_defconfig and thus exposed it.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>