netdev - Re: netpoll + xmit

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Message-ID: <20090731125654.GB18303@hmsreliant.think-freely.org>
Date:	Fri, 31 Jul 2009 08:56:54 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Matt Mackall <mpm@...enic.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Matt Carlson <mcarlson@...adcom.com>
Subject: Re: netpoll + xmit_lock == deadlock

On Fri, Jul 31, 2009 at 09:30:17AM +0800, Herbert Xu wrote:
> On Wed, Jul 29, 2009 at 09:06:39PM -0400, Neil Horman wrote:
> >
> > but nothing in that path takes the xmit_lock.  The poll_lock is taken in that
> > patch, but thats for recieve_side syncronization, not transmit side.  Nothing in
> > the tg3 driver (to use your example), that I can see takes the xmit_lock in the
> > rx path either, so I'm not really sure where you comming from here.
> 
> tg3_poll => tg3_poll_work => tg3_tx => netif_tx_lock

Oh, goodness, thats just asking for disaster.  Setting asside the netpoll issue
for the moment, what if we take an rx interrupt on a cpu while in the middle of
sending a frame?  Whats to stop the NET_RX_SOFTIRQ after the hard interrupt and
recursively taking the _xmit_lock?  With or without netpoll, that seems prone to
deadlock.

Neil

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