[RFC 33/43] PKRAM: atomically add and remove link pages

[Anthony Yznaga <anthony.yznaga@oracle.com>]

Add and remove pkram_link pages from a pkram_obj atomically to prepare
for multithreading.

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
 mm/pkram.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/mm/pkram.c b/mm/pkram.c
index 5f4e4d12865f..042c14dedc25 100644
--- a/mm/pkram.c
+++ b/mm/pkram.c
@@ -551,22 +551,31 @@ static void pkram_truncate(void)
 
 static void pkram_add_link(struct pkram_link *link, struct pkram_obj *obj)
 {
-	link->link_pfn = obj->link_pfn;
-	obj->link_pfn = page_to_pfn(virt_to_page(link));
+	__u64 link_pfn = page_to_pfn(virt_to_page(link));
+	__u64 *head = &obj->link_pfn;
+
+	do {
+		link->link_pfn = *head;
+	} while (cmpxchg64(head, link->link_pfn, link_pfn) != link->link_pfn);
 }
 
 static struct pkram_link *pkram_remove_link(struct pkram_obj *obj)
 {
 	struct pkram_link *current_link;
+	__u64 *head = &obj->link_pfn;
+	__u64 head_pfn = *head;
+
+	while (head_pfn) {
+		current_link = pfn_to_kaddr(head_pfn);
+		if (cmpxchg64(head, head_pfn, current_link->link_pfn) == head_pfn) {
+			current_link->link_pfn = 0;
+			return current_link;
+		}
 
-	if (!obj->link_pfn)
-		return NULL;
-
-	current_link = pfn_to_kaddr(obj->link_pfn);
-	obj->link_pfn = current_link->link_pfn;
-	current_link->link_pfn = 0;
+		head_pfn = *head;
+	}
 
-	return current_link;
+	return NULL;
 }
 
 static void pkram_stream_init(struct pkram_stream *ps,
-- 
2.13.3