Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

[Daniel Colascione]

On Sat, Jul 14, 2018 at 11:18 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> On Tue, Jul 10, 2018 at 08:40:19PM -0700, Alexei Starovoitov wrote:
> [..]
>> > The kernel program might do:
>> >
>> > =====
>> > const int current_map_key = 1;
>> > void *current_map = bpf_map_lookup_elem(outer_map, &current_map_key);
>> >
>> > int stats_key = 42;
>> > uint64_t *stats_value = bpf_map_lookup_elem(current_map, &stats_key);
>> > __sync_fetch_and_add(&stats_value, 1);
>> > =====
>> >
>> > If a userspace does:
>> >
>> > 1. Write new fd to outer_map[1].
>> > 2. Call BPF_SYNC_MAP_ACCESS.
>> > 3. Start deleting everything in the old map.
>> >
>> > How can we guarantee that the __sync_fetch_and_add will not add to the
>> > old map?
>>
>> without any changes to the kernel sys_membarrier will work.
>> And that's what folks use already.
>> BPF_SYNC_MAP_ACCESS implemented via synchronize_rcu() will work
>> as well whether in the current implementation where rcu_lock/unlock
>> is done outside of the program and in the future when
>> rcu_lock/unlock are called by the program itself.
>
> Cool Alexei and Lorenzo, sounds great to me. Daniel want to send a follow up
> patch with BPF_SYNC_MAP_ACCESS changes then?

Will do. Mind if I just mine this thread for the doc comment?