s390/mm: add set_pXd()/set_pte() helper functions

Add set_pXd()/set_pte() helper functions which must be used to update
page table entries. The new helpers use WRITE_ONCE() to make sure that
a page table entry is written to only once.

Without this the compiler could otherwise generate code which writes
several times to a page table entry when updating its contents from
invalid to valid, which could lead to surprising results especially
for multithreaded processes...
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -881,6 +881,31 @@ static inline pgprot_t pte_pgprot(pte_t pte)
  * pgd/pmd/pte modification functions
  */
 
+static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
+{
+	WRITE_ONCE(*pgdp, pgd);
+}
+
+static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
+{
+	WRITE_ONCE(*p4dp, p4d);
+}
+
+static inline void set_pud(pud_t *pudp, pud_t pud)
+{
+	WRITE_ONCE(*pudp, pud);
+}
+
+static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
+{
+	WRITE_ONCE(*pmdp, pmd);
+}
+
+static inline void set_pte(pte_t *ptep, pte_t pte)
+{
+	WRITE_ONCE(*ptep, pte);
+}
+
 static inline void pgd_clear(pgd_t *pgd)
 {
 	if ((pgd_val(*pgd) & _REGION_ENTRY_TYPE_MASK) == _REGION_ENTRY_TYPE_R1)