Re: [PATCH v2 nf-next 4/7] netfilter: conntrack: add gc worker to remove timed-out entries

[Florian Westphal <fw@strlen.de>]

Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2016-08-24 at 22:11 +0200, Florian Westphal wrote:
> > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > On Wed, 2016-08-24 at 13:55 +0200, Florian Westphal wrote:
> > > > Conntrack gc worker to evict stale entries.
> > > 
> > > 
> > > >  static struct nf_conn *
> > > >  __nf_conntrack_alloc(struct net *net,
> > > >  		     const struct nf_conntrack_zone *zone,
> > > > @@ -1527,6 +1597,7 @@ static int untrack_refs(void)
> > > >  
> > > >  void nf_conntrack_cleanup_start(void)
> > > >  {
> > > > +	conntrack_gc_work.exiting = true;
> > > >  	RCU_INIT_POINTER(ip_ct_attach, NULL);
> > > >  }
> > > >  
> > > > @@ -1536,6 +1607,9 @@ void nf_conntrack_cleanup_end(void)
> > > >  	while (untrack_refs() > 0)
> > > >  		schedule();
> > > >  
> > > > +	cancel_delayed_work_sync(&conntrack_gc_work.dwork);
> > > > +	/* can be re-scheduled once */
> > > 
> > > Are you sure ?
> > > 
> > > As conntrack_gc_work.exiting = true, I do not see how this can happen ?
> > 
> > nf_conntrack_cleanup_start() sets exiting = true
> > 
> > current cpu blocks in
> > 
> > cancel_delayed_work_sync(&conntrack_gc_work.dwork);
> > 
> > Iff the work queue was running on other cpu but was already past
> > gc_work->exiting check then when cancel_delayed_work_sync() (first one)
> > returns it will have re-armed itself via schedule_delayed_work().
> > 
> > So I think the 2nd cancel_delayed_work_sync is needed.
> > 
> > Let me know if you'd like to see a v3 with more verbose
> > comment about this.
> 
> If you were using cancel_delayed_work() (instead of
> cancel_delayed_work_sync()) I would understand the concern.
> 
> But here you are using the thing that was designed to exactly avoid the
> issue, of both work running on another cpu and/or re-arming itself.
> 
> If what you are saying was true, we would have to fix hundreds of
> cancel_delayed_work_sync() call sites ...

Ok, I see, the _sync version indeed seems to be desgined to
suppress/avoid re-arming.

I will send a v3 without the 2nd cancel_delayed_work_sync, thanks Eric!

(I will preserve/add your Acked-by tags to the unchanged patches so you
 won't need to resend them).