[RFC v2 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 | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/mm/pkram.c b/mm/pkram.c
index 08144c18d425..382ccf6f789f 100644
--- a/mm/pkram.c
+++ b/mm/pkram.c
@@ -535,33 +535,42 @@ static void pkram_truncate(void)
 static void pkram_add_link(struct pkram_link *link, struct pkram_data_stream 
*pds)
 {
        __u64 link_pfn = page_to_pfn(virt_to_page(link));
+       __u64 *tail = pds->tail_link_pfnp;
+       __u64 tail_pfn;
 
-       if (!*pds->head_link_pfnp) {
+       do {
+               tail_pfn = *tail;
+       } while (cmpxchg64(tail, tail_pfn, link_pfn) != tail_pfn);
+
+       if (!tail_pfn) {
                *pds->head_link_pfnp = link_pfn;
-               *pds->tail_link_pfnp = link_pfn;
        } else {
-               struct pkram_link *tail = pfn_to_kaddr(*pds->tail_link_pfnp);
+               struct pkram_link *prev_tail = pfn_to_kaddr(tail_pfn);
 
-               tail->link_pfn = link_pfn;
-               *pds->tail_link_pfnp = link_pfn;
+               prev_tail->link_pfn = link_pfn;
        }
 }
 
 static struct pkram_link *pkram_remove_link(struct pkram_data_stream *pds)
 {
-       struct pkram_link *link;
+       __u64 *head = pds->head_link_pfnp;
+       __u64 head_pfn = *head;
 
-       if (!*pds->head_link_pfnp)
-               return NULL;
+       while (head_pfn) {
+               struct pkram_link *link = pfn_to_kaddr(head_pfn);
 
-       link = pfn_to_kaddr(*pds->head_link_pfnp);
-       *pds->head_link_pfnp = link->link_pfn;
-       if (!*pds->head_link_pfnp)
-               *pds->tail_link_pfnp = 0;
-       else
-               link->link_pfn = 0;
+               if (cmpxchg64(head, head_pfn, link->link_pfn) == head_pfn) {
+                       if (!*head)
+                               *pds->tail_link_pfnp = 0;
+                       else
+                               link->link_pfn = 0;
+                       return link;
+               }
 
-       return link;
+               head_pfn = *head;
+       }
+
+       return NULL;
 }
 
 static struct pkram_link *pkram_new_link(struct pkram_data_stream *pds, gfp_t 
gfp_mask)
-- 
1.8.3.1