Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred expansion/shrinking

[Patrick McHardy <kaber@trash.net>]

On 16.01, Pablo Neira Ayuso wrote:
> On Fri, Jan 16, 2015 at 07:35:57PM +0000, Patrick McHardy wrote:
> > On 16.01, Thomas Graf wrote:
> > > On 01/16/15 at 06:36pm, Patrick McHardy wrote:
> > > > 
> > > > Well, we do have a problem with interrupted dumps. As you know once
> > > > the netlink message buffer is full, we return to userspace and
> > > > continue dumping during the next read. Expanding obviously changes
> > > > the order since we rehash from bucket N to N and 2N, so this will
> > > > indeed cause duplicate (doesn't matter) and missed entries.
> > > 
> > > Right,but that's a Netlink dump issue and not specific to rhashtable.
> > 
> > Well, rhashtable (or generally resizing) will make it a lot worse.
> > Usually we at worst miss entries which were added during the dump,
> > which is made up by the notifications.
> > 
> > With resizing we might miss anything, its completely undeterministic.
> > 
> > > Putting the sequence number check in place should be sufficient
> > > for sets, right?
> > 
> > I don't see how. The problem is that the ordering of the hash changes
> > and it will skip different entries than those that have already been
> > dumped.
> 
> I think the generation counter should catch up this sort of problems.
> The resizing is triggered by a new/deletion element, which bumps it
> once the transaction is handled.

I don't think so, it tracks only two generations, we can have an
arbitrary amount of changes while performing a dump.