Subject: Re: RFC: netdev: allow ethtool physical id to drop rtnl_lock

From: "Michael Chan" <[email-masked]>
Date: Sat, 31 Oct 2009 09:44:22 -0700

On Fri, 2009-10-30 at 10:42 -0700, Stephen Hemminger wrote:
> The ethtool operation to blink LED can take an indeterminately long time,
> blocking out other operations (via rtnl_lock). This patch is an attempt
> to work around the problem.
> 
> It does need more discussion, because it will mean that drivers that formerly
> were protected from changes during blink aren't.  For example, user could
> start device blinking, and then plug in cable causing change netlink event
> to change state or pull cable and have device come down.

Yeah, the biggest concern is shutting down the device while it is still
blinking.  During shutdown, some devices are brought to low power state
and the chip will no longer respond to I/Os to blink the LEDs.  On some
systems, this can cause bus hang or NMI.

> 
> The other possibility is to do this on a driver by driver basis
> which is more effort.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> 
> --- a/net/core/ethtool.c	2009-10-30 10:27:23.621917624 -0700
> +++ b/net/core/ethtool.c	2009-10-30 10:35:53.787670774 -0700
> @@ -17,6 +17,7 @@
>  #include <linux/errno.h>
>  #include <linux/ethtool.h>
>  #include <linux/netdevice.h>
> +#include <linux/rtnetlink.h>
>  #include <asm/uaccess.h>
>  
>  /*
> @@ -781,6 +782,8 @@ static int ethtool_get_strings(struct ne
>  static int ethtool_phys_id(struct net_device *dev, void __user *useraddr)
>  {
>  	struct ethtool_value id;
> +	int err;
> +	static int busy;
>  
>  	if (!dev->ethtool_ops->phys_id)
>  		return -EOPNOTSUPP;
> @@ -788,7 +791,21 @@ static int ethtool_phys_id(struct net_de
>  	if (copy_from_user(&id, useraddr, sizeof(id)))
>  		return -EFAULT;
>  
> -	return dev->ethtool_ops->phys_id(dev, id.data);
> +	if (busy)
> +		return -EBUSY;
> +
> +	/* This operation may take a long time, drop lock */
> +	busy = 1;
> +	dev_hold(dev);
> +	rtnl_unlock();
> +
> +	err = dev->ethtool_ops->phys_id(dev, id.data);
> +
> +	rtnl_lock();
> +	dev_put(dev);
> +	busy = 0;
> +
> +	return err;
>  }
>  
>  static int ethtool_get_stats(struct net_device *dev, void __user *useraddr)
> --
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