Re: [PATCH 1/2] rhashtable: Introduce rhashtable_walk_*

[Thomas Graf <tgraf@suug.ch>]

On 01/27/15 at 10:23pm, Herbert Xu wrote:
> On Tue, Jan 27, 2015 at 11:16:04AM +0000, Thomas Graf wrote:
> > 
> > No objection. I have a patch prepared which allows the user to
> > provide ht->mutex himself so nfset can provide its own existing
> > mutex to rhashtable and lock out the resizes from inserts,
> > removals and dump iterations automatically That would restore the
> > old behaviour of the nfset API without major surgery.
> 
> If you take the mutex you might as well just make it synchronous.
> There is zero difference.
> 
> Maybe you misunderstood my email.  I'm not making it synchronous
> for everybody, just those that always take a lock on inserts/removals
> and therefore don't need per-bucket locks.

Understood. No objection, happy to review patches. I initially did
keep sync/async separate and it lead to considerable code duplication
and complexity. I figured that if a user needs sync insert they could
provide their own locking. I missed to allow controlling the async
resize though. Again, feel free to give a shot, no objections.

This is unrelated to resize run control though, the reason is that
I'm converting tcp_hashinfo et al and they require a hybrid approach.
The tables may be too big to construct a parallel data structure, we
don't want to hold off inserts or deletes while the expensive dump
is underway. Even though we can't build a shadow structure while
locking everybody else out, we still want to provide a way to somehow
achieve consistent information. I think that NLM_F_INTR with fallback
to restarting the dump is a good option and very easy to implement. In
that case, we want to lock out resize from dumping iterations but
still allow parallel insert/delete.