x86, hotplug: Move WBINVD back outside the play_dead loop

On processors with hyperthreading, when only one thread is offlined
the other thread can cause a spurious wakeup on the idled thread.  We
do not want to re-WBINVD when that happens.

Ideally, we should simply skip WBINVD unless we're the last thread on
a particular core to shut down, but there might be similar issues
elsewhere in the system.

Thus, revert to previous behavior of only WBINVD outside the loop.
Partly as a result, remove the mb()'s around it: they are not
necessary since wbinvd() is a serializing instruction, but they were
intended to make sure the compiler didn't do any funny loop
optimizations.

Reported-by: Asit Mallick <asit.k.mallick@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Arjan van de Ven <arjan@linux.kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.hl>
LKML-Reference: <tip-ea53069231f9317062910d6e772cca4ce93de8c8@git.kernel.org>

arch/x86/kernel/smpboot.c

@@ -1422,9 +1422,9 @@ static inline void mwait_play_dead(void)
 			(highest_subcstate - 1);
 	}
 
+	wbinvd();
+
 	while (1) {
-		mb();
-		wbinvd();
 		__monitor(&current_thread_info()->flags, 0, 0);
 		mb();
 		__mwait(eax, 0);
@@ -1433,11 +1433,10 @@ static inline void mwait_play_dead(void)
 
 static inline void hlt_play_dead(void)
 {
+	if (current_cpu_data.x86 >= 4)
+		wbinvd();
+
 	while (1) {
-		mb();
-		if (current_cpu_data.x86 >= 4)
-			wbinvd();
-		mb();
 		native_halt();
 	}
 }