x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers

Use static_cpu_has() to conditionalize the CLFLUSH workaround, and add
memory barriers around it since the documentation is explicit that
CLFLUSH is only ordered with respect to MFENCE.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Link: http://lkml.kernel.org/r/CA%2B55aFzGxcML7j8CEvQPYzh0W81uVoAAVmGctMOUZ7CZ1yYd2A@mail.gmail.com

arch/x86/include/asm/mwait.h

@@ -43,8 +43,11 @@ static inline void __mwait(unsigned long eax, unsigned long ecx)
 static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 {
 	if (!current_set_polling_and_test()) {
-		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
+		if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
+			mb();
 			clflush((void *)&current_thread_info()->flags);
+			mb();
+		}
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
 		if (!need_resched())