netdev - Re: netpoll + xmit

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Message-Id: <20090803.211905.100178246.davem@davemloft.net>
Date:	Mon, 03 Aug 2009 21:19:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mpm@...enic.com
Cc:	bhutchings@...arflare.com, herbert@...dor.apana.org.au,
	netdev@...r.kernel.org, mcarlson@...adcom.com
Subject: Re: netpoll + xmit_lock == deadlock

From: Matt Mackall <mpm@...enic.com>
Date: Mon, 03 Aug 2009 14:59:41 -0500

> First, I don't think we can solve the problem of 'reliably deliver
> printks inside the TX path'. If the driver needs to printk here, odds
> are good that sending isn't possible.

Agreed.

> So I think we should be clear up front that the solution we're looking
> for is 'don't crash the box when trying to printk from the tx path'.
> 
> I think the most straightforward, driver-agnostic way to do this
> is to have netpoll wrap a driver's TX entrypoint such that recursion is
> disabled while in that path. This might actually simplify netpoll's
> internal locking ugliness. Something like:
> 
> int netpoll_xmit_wrapper(struct sk_buff *skb, struct net_device *dev)
> {
> 	struct netpoll_info *npinfo = dev->npinfo;
> 	int ret;
> 
> 	npinfo->recurse += 1; /* add appropriate locking */
> 	ret = npinfo->orig_tx(skb, dev);
> 	npinfo->recurse -= 1;
> 	
> 	return ret;
> }

Looks workable.  Probably we even hold the netpoll lock here so
no additional protection would even be needed.
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