netdev - Re: netpoll + xmit

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Message-ID: <20090729194300.GB17410@hmsreliant.think-freely.org>
Date:	Wed, 29 Jul 2009 15:43:00 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Matt Carlson <mcarlson@...adcom.com>
Subject: Re: netpoll + xmit_lock == deadlock

On Wed, Jul 29, 2009 at 02:07:58PM -0500, Matt Mackall wrote:
> On Wed, 2009-07-29 at 15:35 +0800, Herbert Xu wrote:
> > Hi:
> > 
> > While working on TX mitigiation, I noticed that while netpoll
> > takes care to avoid recursive dead locks on the NAPI path, it
> > has no protection against the TX path when calling the poll
> > function.
> > 
> > So if a driver is in the TX path, and a printk occurs, then a
> > recursive dead lock can occur if that driver tries to take the
> > xmit lock in its poll function to clean up descriptors.
> > 
> > Fortunately not a lot of drivers do this but at least some are
> > vulnerable to it, e.g., tg3.
> > 
> > So we need to make it very clear that the poll function must
> > not take any locks or they must use try_lock if the driver is
> > to support netpoll.
> 
> What do you propose?

I think there is actually some recursion protection.  If you look in
netpoll_send_skb (where all netpoll transmits pass through), we do a
__netif_tx_trylock, and only continue down the tx path if we obtain the lock.
If not, we call netpoll_poll, wait a while, and try again.  I think that should
prevent the deadlock condition you are concerned about.

Regards
Neil (who hates netpoll). :)

> 
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