s390: avoid using global register for current_stack_pointer

Commit 30de14b188 ("s390: current_stack_pointer shouldn't be a
function") made current_stack_pointer a global register variable like
on many other architectures. Unfortunately on s390 it uncovers old
gcc bug which is fixed only since gcc-9.1 [gcc commit 3ad7fed1cc87
("S/390: Fix PR89775. Stackpointer save/restore instructions removed")]
and backported to gcc-8.4 and later. Due to this bug gcc versions prior
to 8.4 generate broken code which leads to stack corruptions.

Current minimal gcc version required to build the kernel is declared
as 5.1. It is not possible to fix all old gcc versions, so work
around this problem by avoiding using global register variable for
current_stack_pointer.

Fixes: 30de14b188 ("s390: current_stack_pointer shouldn't be a function")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/include/asm/processor.h

@@ -199,7 +199,16 @@ unsigned long __get_wchan(struct task_struct *p);
 /* Has task runtime instrumentation enabled ? */
 #define is_ri_task(tsk) (!!(tsk)->thread.ri_cb)
 
-register unsigned long current_stack_pointer asm("r15");
+/* avoid using global register due to gcc bug in versions < 8.4 */
+#define current_stack_pointer (__current_stack_pointer())
+
+static __always_inline unsigned long __current_stack_pointer(void)
+{
+	unsigned long sp;
+
+	asm volatile("lgr %0,15" : "=d" (sp));
+	return sp;
+}
 
 static __always_inline unsigned short stap(void)
 {