Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2) command

[Alexei Starovoitov]

On Tue, Aug 14, 2018 at 01:37:12PM -0700, Daniel Colascione wrote:
> 
> > If we agree on that, should bpf_map_update handle it then?
> > Wouldn't it be much easier to understand and use from user pov?
> > No new commands to learn. map_update syscall replaced the map
> > and old map is no longer accessed by the program via this given map-in-map.
> 
> Maybe with a new BPF_SYNCHRONIZE flag for BPF_MAP_UPDATE_ELEM and
> BPF_MAP_DELETE_ELEM. Otherwise, it seems wrong to make every user of
> these commands pay for synchronization that only a few will need.

I don't think extra flag is needed. Extra sync_rcu() for map-in-map
is useful for all users. I would consider it a bugfix,
since users that examine deleted map have this race today
and removing the race is always a good thing especially since the cost
is small.