netdev - Re: [PATCH v8 04/11] net/mlx4_en: add support for fast rx drop bpf program

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Date:	Mon, 18 Jul 2016 15:07:01 +0200
From:	Tom Herbert <tom@...bertland.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Brenden Blanco <bblanco@...mgrid.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Saeed Mahameed <saeedm@....mellanox.co.il>,
	Martin KaFai Lau <kafai@...com>, Ari Saha <as754m@....com>,
	Or Gerlitz <gerlitz.or@...il.com>,
	john fastabend <john.fastabend@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v8 04/11] net/mlx4_en: add support for fast rx drop bpf program

On Mon, Jul 18, 2016 at 2:48 PM, Thomas Graf <tgraf@...g.ch> wrote:
> On 07/18/16 at 01:39pm, Tom Herbert wrote:
>> On Mon, Jul 18, 2016 at 11:10 AM, Thomas Graf <tgraf@...g.ch> wrote:
>> > I agree with that but I would like to keep the current per net_device
>> > atomic properties.
>>
>> I don't see that see that there is any synchronization guarantees
>> using xchg. For instance, if the pointer is set right after being read
>> by a thread for one queue and right before being read by a thread for
>> another queue, this could result in the old and new program running
>> concurrently or old one running after new. If we need to synchronize
>> the operation across all queues then sequence
>> ifdown,modify-config,ifup will work.
>
> Right, there are no synchronization guarantees between threads and I
> don't think that's needed. The guarantee that is provided is that if
> I replace a BPF program, the replace either succeeds in which case
> all packets have been either processed by the old or new program. Or
> the replace failed in which case the old program was left intact and
> all packets are still going through the old program.
>
> This is a nice atomic replacement principle which would be nice to
> preserve.

Sure, if replace operation fails then old program should remain in
place. But xchg can't fail, so it seems like part is just giving a
false sense of security that program replacement is somehow
synchronized across queues.

Tom