Re: [PATCH 1/7] Make restore_obj() tolerate a preexisting object in the hash (v2)

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

SH> I don't think this part of the comment is quite right here.  The
SH> "on failure this cleans up the object itself" really is for the
SH> ref_drop under IS_ERR() check below.

SH> The ref_drop here is for the ref taken by obj_new(), which is only
SH> done in this path of course.

As just discussed on IRC, I couldn't really correct the comment
because the logic it was describing was incorrect.  Below is a version
with the correct comment *and* logic :)

-- 
Dan Smith
IBM Linux Technology Center
email: danms-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org

Make restore_obj() tolerate a preexisting object in the hash (v3)

... 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 v3:
 - Fix the logic in the case where we need to do an obj_new() and fail,
   so that we don't do ref_drop() twice

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>

diff --git a/checkpoint/objhash.c b/checkpoint/objhash.c
index 7208382..3b360cb 100644
--- a/checkpoint/objhash.c
+++ b/checkpoint/objhash.c
@@ -1064,17 +1064,32 @@ 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 = ERR_PTR(-EINVAL);
-	else
+	obj = obj_find_by_objref(ctx, h->objref);
+	if (!obj) {
 		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 to balance the one taken by obj_new()
+		 */
+		if (!IS_ERR(obj))
+			ops->ref_drop(ptr, 0);
+	} else if ((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.  If either of those don't match,
+		 * we fail.
+		 */
+		obj = ERR_PTR(-EINVAL);
+	}
+
 	if (IS_ERR(obj)) {
+		/* This releases our final reference on the object
+		 * returned by ops->restore()
+		 */
 		ops->ref_drop(ptr, 1);
 		return PTR_ERR(obj);
 	}