[PATCH 1/2] Make restore_obj() tolerate a preexisting object in the hash (v2)

[Dan Smith <danms-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

... as long as the pointer is the same as that returned from the restore
function.  Also move the compulsory ref_drop() so that it only gets
done if we created the new object.

The existing object tolerance is important for netdev restore because it
means that I can refer to a peer by its objref instead of needing the
(previously-rejected) veth_peer() function.  If this is not acceptable,
then I'll need to keep a separate list of pairs.

Changes in v2:
 - Check that the type of the object already in the hash matches that
   of the objref header we're reading.
 - Add a comment about why and how we might get into this sort of
   situation.

Signed-off-by: Dan Smith <danms-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
---
 checkpoint/objhash.c |   27 +++++++++++++++++++--------
 1 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/checkpoint/objhash.c b/checkpoint/objhash.c
index 0b06b06..52827a4 100644
--- a/checkpoint/objhash.c
+++ b/checkpoint/objhash.c
@@ -1059,16 +1059,27 @@ int restore_obj(struct ckpt_ctx *ctx, struct ckpt_hdr_objref *h)
 	if (IS_ERR(ptr))
 		return PTR_ERR(ptr);
 
-	if (obj_find_by_objref(ctx, h->objref))
+	obj = obj_find_by_objref(ctx, h->objref);
+	if (obj && ((obj->ptr != ptr) || (obj->ops->obj_type != h->objtype))) {
+		/* Normally, we expect an object to not already exist
+		 * in the hash.  However, for some special scenarios
+		 * where we're restoring sets of objects that must be
+		 * co-allocated (such, as veth netdev pairs) we need
+		 * to tolerate this case if the second restore returns
+		 * the correct type and pointer, as specified in the
+		 * existing object.  Otherwise, we fail.
+		 */
 		obj = ERR_PTR(-EINVAL);
-	else
+	} else {
 		obj = obj_new(ctx, ptr, h->objref, h->objtype);
-	/*
-	 * Drop an extra reference to the object returned by ops->restore:
-	 * On success, this clears the extra reference taken by obj_new(),
-	 * and on failure, this cleans up the object itself.
-	 */
-	ops->ref_drop(ptr, 0);
+		/*
+		 * Drop an extra reference to the object returned by
+		 * ops->restore: On success, this clears the extra
+		 * reference taken by obj_new(), and on failure, this
+		 * cleans up the object itself.
+		 */
+		ops->ref_drop(ptr, 0);
+	}
 	if (IS_ERR(obj)) {
 		ops->ref_drop(ptr, 1);
 		return PTR_ERR(obj);
-- 
1.6.2.5