I have the following link I use to consult when I need to remember anything regarding inline assembler.
http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html
I can't guarrantee it's completely correct but there is the following phrase in it: "If our instruction can alter the 
condition code register, we have to add "cc" to the list of clobbered registers.". This means that if assembler block 
modifies EFLAGS (or similar registers on other architectures) "cc" needs to be added to clobber list.
Now the question is: does not the __cpu_membarrier() implementation (which is citated below) lack "cc" in clobber list?

#define __cpu_membarrier()                                              \
    ({                                                                  \
        extern unsigned __cpu_flags;                                    \
        /* check for X86_CPU_SSE2 */                                    \
        if (__cpu_flags & (1 << 15)) {                                  \
            __asm__ __volatile__ ("mfence");                            \
        } else {                                                        \
            __asm__ __volatile__ ("lock; orb $0,0(%esp)");              \
        }                                                               \
    })