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

[Anthony Yznaga]

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

Signed-off-by: Anthony Yznaga <anthony.yzn...@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