netdev - Re: [patch net-next] net: squash ->rx_handler and ->rx_handler

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Date:	Sat, 30 Mar 2013 09:23:08 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	fubar@...ibm.com, andy@...yhouse.net, kaber@...sh.net,
	stephen@...workplumber.org, jesse@...ira.com,
	alexander.h.duyck@...el.com, xiyou.wangcong@...il.com,
	dev@...nvswitch.org, rostedt@...dmis.org,
	nicolas.2p.debian@...il.com, tglx@...utronix.de,
	streeter@...hat.com, paulmck@...ibm.com, ivecera@...hat.com
Subject: Re: [patch net-next] net: squash ->rx_handler and ->rx_handler_data
 into single rcu pointer

On Sat, 2013-03-30 at 16:57 +0100, Jiri Pirko wrote:
> No need to have two pointers in struct netdevice for rx_handler func and
> priv data. Just embed rx_handler structure into driver port_priv and
> have ->func pointer there. This introduces no performance penalty,
> reduces struct netdevice by one pointer and reduces number of needed
> rcu_dereference calls from 2 to 1.
> 

Thats not true.

> Note this also fixes the race bug pointed out by Steven Rostedt and
> fixed by patch "[PATCH] net: add a synchronize_net() in
> netdev_rx_handler_unregister()" by Eric Dumazet. This is based on
> current net-next tree where the patch is not applied yet.
> I can rebase it on whatever tree/state, just say so.
> 
> Smoke tested with all drivers who use rx_handler.
> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Jiri Pirko <jiri@...nulli.us>
> ---

I see no value for this patch.

It obfuscates things for no good reason.

Once again rcu_dereference(dev->field) has no cost, its a memory read,
like dev->field.

I fear you don't understand enough RCU to make so invasive changes.

Your patch adds a double dereference on fast path, and its more
expensive than two single deref.

dev->rx_handler actually gets the function pointer, and after your patch
we would have to do dev->rx_handler->func instead, which is bad on many
cpus.

I'll send a patch reordering some fields of net_device, because as time
passed, it seems a lot of junk broke work done in commit
cd13539b8bc9ae884 (net: shrinks struct net_device)

offsetof(struct net_device,dev_addr)=0x258
offsetof(struct net_device,rx_handler)=0x2b8
offsetof(struct net_device,ingress_queue)=0x2c8
offsetof(struct net_device,broadcast)=0x278


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