netdev - Re: [PATCH net-next V3 4/4] net/sched: Introduce act_tunnel

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Date:   Tue, 30 Aug 2016 05:27:52 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Amir Vadai <amir@...ai.me>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Hadar Hen Zion <hadarh@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...lanox.com>, Jiri Benc <jbenc@...hat.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Shmulik Ladkani <shmulik.ladkani@...il.com>,
        Tom Herbert <tom@...bertland.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Amir Vadai <amirva@...lanox.com>
Subject: Re: [PATCH net-next V3 4/4] net/sched: Introduce act_tunnel_key

On Tue, 2016-08-30 at 14:39 +0300, Amir Vadai wrote:
> On Tue, Aug 30, 2016 at 02:03:08PM +0300, Amir Vadai wrote:

> > Regarding the specific action in this patchset, correct me if I'm wrong,
> > but I think that the lock could be removed safely.

Sure ;)

> > 
> > When the action is modified during traffic, an existing tcf_enc_metadata
> > is not changed, but a new metadata is allocated and the pointer is
> > replaced to point to the new one.
> > I just need to make sure that when changing an action from 'release'
> > into 'set' - tcf_enc_metadata will be set before the action type is
> > changed - change the order of operations and add a memory barrier.
> > Here is a pseudo code to explain:
> > 
> > metadata_new = new allocated metadata
> > metadata_old = t->tcft_enc_metadata
> > 
> 
> Oh - I had a typo here:
> Need to set the metadata and only after that, set the action:
> 
> t->tcft_enc_metadata = metadata_new
> wmb()

rcu_assign_pointer() is your friend, it auto documents the thing.


Note that you probably need to store in the allocated object :

	dst,
	tcf_action (a copy of it, read in tunnel_key_act()
	tcft_action ( a copy of it, read in tunnel_key_act())
	rcu_head rcu for kfree_rcu()

> t->tcft_action = encapdecap
> 
> > t->tcft_action = encapdecap
> > 
> > /* make sure the compiler won't swap the setting of tcft_action with
> >  * tcft_enc_metadata
> >  */
> > wmb()
> > 
> > t->tcft_enc_metadata = metadata_new
> > release metadata_old
> > 
> > 
> > This way, no need for lock between the init() and act() operations.
> > 
> > Please let me know if you see a problem with this approach.
> > I will also change the stats to be percpu.

Right, check tcf_hash_create() last argument. (false -> true)

Thanks.