s390/spinlock: add gmb memory barrier

CVE-2017-5753
CVE-2017-5715
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>

arch/s390/include/asm/barrier.h

@@ -7,6 +7,8 @@
 #ifndef __ASM_BARRIER_H
 #define __ASM_BARRIER_H
 
+#include <asm/alternative.h>
+
 /*
  * Force strict CPU ordering.
  * And yes, this is required on UP too when we're talking
@@ -22,6 +24,14 @@
 
 #define mb() do {  asm volatile(__ASM_BARRIER : : : "memory"); } while (0)
 
+static inline void gmb(void)
+{
+	asm volatile(
+		ALTERNATIVE("", ".long 0xb2e8f000", 81)
+		: : : "memory");
+}
+#define gmb gmb
+
 #define rmb()				barrier()
 #define wmb()				barrier()
 #define dma_rmb()			mb()