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@xxxxxxxxxx
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@xxxxxxxxxx>
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);
}
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