xen-blkback: implement safe iterator for the list of persistent grants

Change foreach_grant iterator to a safe version, that allows freeing
the element while iterating. Also move the free code in
free_persistent_gnts to prevent freeing the element before the rb_next
call.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad@kernel.org>
Cc: xen-devel@lists.xen.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 11 insertions, 7 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 74374fb762aa..5ac841ff6cc7 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -161,10 +161,12 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 static void make_response(struct xen_blkif *blkif, u64 id,
                           unsigned short op, int st);
 
-#define foreach_grant(pos, rbtree, node) \
-        for ((pos) = container_of(rb_first((rbtree)), typeof(*(pos)), node); \
+#define foreach_grant_safe(pos, n, rbtree, node) \
+        for ((pos) = container_of(rb_first((rbtree)), typeof(*(pos)), node), \
+             (n) = rb_next(&(pos)->node); \
              &(pos)->node != NULL; \
-             (pos) = container_of(rb_next(&(pos)->node), typeof(*(pos)), node))
+             (pos) = container_of(n, typeof(*(pos)), node), \
+             (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL)
 
 
 static void add_persistent_gnt(struct rb_root *root,
@@ -217,10 +219,11 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
         struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
         struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
         struct persistent_gnt *persistent_gnt;
+        struct rb_node *n;
         int ret = 0;
         int segs_to_unmap = 0;
 
-        foreach_grant(persistent_gnt, root, node) {
+        foreach_grant_safe(persistent_gnt, n, root, node) {
                 BUG_ON(persistent_gnt->handle ==
                         BLKBACK_INVALID_HANDLE);
                 gnttab_set_unmap_op(&unmap[segs_to_unmap],
@@ -230,9 +233,6 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
                         persistent_gnt->handle);
 
                 pages[segs_to_unmap] = persistent_gnt->page;
-                rb_erase(&persistent_gnt->node, root);
-                kfree(persistent_gnt);
-                num--;
 
                 if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST ||
                         !rb_next(&persistent_gnt->node)) {
@@ -241,6 +241,10 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
                         BUG_ON(ret);
                         segs_to_unmap = 0;
                 }
+
+                rb_erase(&persistent_gnt->node, root);
+                kfree(persistent_gnt);
+                num--;
         }
         BUG_ON(num != 0);
 }