s390/extmem: use refcount_t for refcount

Reference counters are preferred to use refcount_t instead of
atomic_t.
This is because the implementation of refcount_t can prevent
overflows and detect possible use-after-free.
So convert atomic_t ref counters to refcount_t.

Link: http://lkml.kernel.org/r/20190808071817.6595-1-hslester96@gmail.comSigned-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/mm/extmem.c

@@ -19,6 +19,7 @@
 #include <linux/memblock.h>
 #include <linux/ctype.h>
 #include <linux/ioport.h>
+#include <linux/refcount.h>
 #include <asm/diag.h>
 #include <asm/page.h>
 #include <asm/pgtable.h>
@@ -64,7 +65,7 @@ struct dcss_segment {
 	char res_name[16];
 	unsigned long start_addr;
 	unsigned long end;
-	atomic_t ref_count;
+	refcount_t ref_count;
 	int do_nonshared;
 	unsigned int vm_segtype;
 	struct qrange range[6];
@@ -362,7 +363,7 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long
 	seg->start_addr = start_addr;
 	seg->end = end_addr;
 	seg->do_nonshared = do_nonshared;
-	atomic_set(&seg->ref_count, 1);
+	refcount_set(&seg->ref_count, 1);
 	list_add(&seg->list, &dcss_list);
 	*addr = seg->start_addr;
 	*end  = seg->end;
@@ -422,7 +423,7 @@ segment_load (char *name, int do_nonshared, unsigned long *addr,
 		rc = __segment_load (name, do_nonshared, addr, end);
 	else {
 		if (do_nonshared == seg->do_nonshared) {
-			atomic_inc(&seg->ref_count);
+			refcount_inc(&seg->ref_count);
 			*addr = seg->start_addr;
 			*end  = seg->end;
 			rc    = seg->vm_segtype;
@@ -468,7 +469,7 @@ segment_modify_shared (char *name, int do_nonshared)
 		rc = 0;
 		goto out_unlock;
 	}
-	if (atomic_read (&seg->ref_count) != 1) {
+	if (refcount_read(&seg->ref_count) != 1) {
 		pr_warn("DCSS %s is in use and cannot be reloaded\n", name);
 		rc = -EAGAIN;
 		goto out_unlock;
@@ -544,7 +545,7 @@ segment_unload(char *name)
 		pr_err("Unloading unknown DCSS %s failed\n", name);
 		goto out_unlock;
 	}
-	if (atomic_dec_return(&seg->ref_count) != 0)
+	if (!refcount_dec_and_test(&seg->ref_count))
 		goto out_unlock;
 	release_resource(seg->res);
 	kfree(seg->res);