netdev - Re: [PATCH] net: dev_getfirstbyhwtype() optimization

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Message-ID: <20100319023256.GD2894@linux.vnet.ibm.com>
Date:	Thu, 18 Mar 2010 19:32:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: dev_getfirstbyhwtype() optimization

On Thu, Mar 18, 2010 at 10:27:25PM +0100, Eric Dumazet wrote:
> Use RCU to avoid RTNL use in dev_getfirstbyhwtype()
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 17b1686..0f2e9fc 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -772,14 +772,17 @@ EXPORT_SYMBOL(__dev_getfirstbyhwtype);
> 
>  struct net_device *dev_getfirstbyhwtype(struct net *net, unsigned short type)
>  {
> -	struct net_device *dev;
> +	struct net_device *dev, *ret = NULL;
> 
> -	rtnl_lock();
> -	dev = __dev_getfirstbyhwtype(net, type);
> -	if (dev)
> -		dev_hold(dev);
> -	rtnl_unlock();
> -	return dev;
> +	rcu_read_lock();
> +	for_each_netdev_rcu(net, dev)
> +		if (dev->type == type) {
> +			dev_hold(dev);
> +			ret = dev;
> +			break;
> +		}
> +	rcu_read_unlock();
> +	return ret;

Looks good, but I don't understand how it helps to introduce the
local variable "ret".

							Thanx, Paul

>  }
>  EXPORT_SYMBOL(dev_getfirstbyhwtype);
> 
> 
> 
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