ARM: why smp_mb() is not needed in the "__mutex_fastpath_lock" and "__mutex_fastpath_unlock" functions

[shan kang]

Hello,
   I wonder why smp_mb() is not needed in the "__mutex_fastpath_lock"
and "__mutex_fastpath_unlock" functions which are located in the
"arch/arm/include/asm/mutex.h"?
   I think "dmb" instruction is necessary there.