netdev - Re: [RFC PATCH net-next] net: sched: flower: refactor reoffload for concurrent access

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Date:   Wed, 17 Apr 2019 09:34:32 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [RFC PATCH net-next] net: sched: flower: refactor reoffload for
 concurrent access

On Wed, 17 Apr 2019 07:29:36 +0000, Vlad Buslov wrote:
> On Wed 17 Apr 2019 at 00:49, Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
> > On Tue, 16 Apr 2019 17:20:47 +0300, Vlad Buslov wrote:  
> >> @@ -1551,6 +1558,10 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
> >>  		goto errout_mask;
> >>
> >>  	if (!tc_skip_hw(fnew->flags)) {
> >> +		spin_lock(&tp->lock);
> >> +		list_add(&fnew->hw_list, &head->hw_filters);
> >> +		spin_unlock(&tp->lock);
> >> +
> >>  		err = fl_hw_replace_filter(tp, fnew, rtnl_held, extack);
> >>  		if (err)
> >>  			goto errout_ht;  
> >
> > Duplicated deletes should be fine, but I'm not sure same is true for
> > adds.  Won't seeing an add with the same cookie twice confuse drivers?
> >
> > There's also the minor issue of offloaded count being off in that
> > case :)  
> 
> Hmmm, okay. Rejecting duplicate cookies should be a trivial change to
> drivers though. Do you see any faults with this approach in general?

Trivial or not it adds up, the stack should make driver authors' job as
easy as possible.  The simplest thing to do would be to add a mutex
around the HW calls.  But that obviously doesn't work for you, cause
you want multiple outstanding requests to the FW for a single tp, right?

How about a RW lock, that would take R on normal add/replace/del paths
and W on replays?  That should scale, no?