Quoting Dan Smith (danms@xxxxxxxxxx):
> ... 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@xxxxxxxxxx>
> ---
> checkpoint/objhash.c | 31 ++++++++++++++++++++++---------
> 1 files changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/checkpoint/objhash.c b/checkpoint/objhash.c
> index 7208382..65875e3 100644
> --- a/checkpoint/objhash.c
> +++ b/checkpoint/objhash.c
> @@ -1064,16 +1064,29 @@ 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: On success, this clears the extra
> + * reference taken by obj_new(), and on failure, this
> + * cleans up the object itself.
I don't think this part of the comment is quite right here. The
"on failure this cleans up the object itself" really is for the
ref_drop under IS_ERR() check below.
The ref_drop here is for the ref taken by obj_new(), which is only
done in this path of course.
> + */
> + 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)) {
Here point out that we are putting the reference taken by
ops->restore(). If obj is not an error, then we keep an
extra ref to pin obj while it is on the hash.
> ops->ref_drop(ptr, 1);
> return PTR_ERR(obj);
-serge
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