Hi Hepeng,
Thanks, now I understand.
On Sat, Feb 27, 2021 at 12:54 AM 贺鹏 <xnhp0...@gmail.com> wrote:
>
> Hi,
>
> Thanks William and Ilya for a detailed revisiting of the origin of the
> problem. I learned a lot.
>
> I now understand that the mix using of RCU and refcounts is not
> intended in the first place.
> But my point is that mix using RCU and refcounts now gives you more
> choices, and actually eases the code changes.
>
> For example, the code for * ofproto_dpif_lookup_by_name* or other
> ofproto lookup function,
> when only using refcounts, you need to change it to:
>
> struct ofproto_dpif *
> ofproto_dpif_lookup_by_uuid(const struct uuid *uuid)
> {
> struct ofproto_dpif *ofproto;
>
> HMAP_FOR_EACH_WITH_HASH (ofproto, all_ofproto_dpifs_by_uuid_node,
> uuid_hash(uuid), &all_ofproto_dpifs_by_uuid) {
> if (uuid_equals(&ofproto->uuid, uuid)) {
>
> ---> if ovs_refcount_try_ref(ofproto)
>
> return ofproto;
> }
> }
> return NULL;
> }
>
> and after finish its usage, you have to do ofproto_unref the ofproto.
>
> This is why I said, most accessing to ofproto is ephemeral. If you
> change to use the pure refcounts solution, you have
> to add refcount and release it every time you access the ofproto. We
> should be more careful and remember to unref
> the ofproto.
>
> However, if using RCU and refcounts, in the above case, you do not
> need to change the code, since the RCU ensures that
> these ephemeral accesses are safe.
How do we know which access is ephemeral, so no refcount is needed,
and which access is not, so we have to add refcount?
>
> you only need to add refcount, when you find that the pointer to
> ofproto lives longer than one grace period.
>
>
> This is why in my patch, I do not add ref to ofproto after its
> creation. I agree the patch is not complete and has issues,
> and understand it could confuse people if changes into mix RCU and
> refcounts version.
>
I see, thanks!
William
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