powerpc/mm: Add memory barrier in __hugepte_alloc()

__hugepte_alloc() uses kmem_cache_zalloc() to allocate a zeroed PTE
and proceeds to use the newly allocated PTE. Add a memory barrier to
make sure that the other CPUs see a properly initialized PTE.

Based on a fix suggested by James Dykman.
Reported-by: NJames Dykman <jdykman@us.ibm.com>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NSukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Tested-by: NJames Dykman <jdykman@us.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hugetlbpage.c

@@ -81,6 +81,13 @@ static int __hugepte_alloc(struct mm_struct *mm, hugepd_t *hpdp,
 	if (! new)
 		return -ENOMEM;
 
+	/*
+	 * Make sure other cpus find the hugepd set only after a
+	 * properly initialized page table is visible to them.
+	 * For more details look for comment in __pte_alloc().
+	 */
+	smp_wmb();
+
 	spin_lock(&mm->page_table_lock);
 #ifdef CONFIG_PPC_FSL_BOOK3E
 	/*