netdev - Re: netpoll + xmit

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Message-ID: <20090730010639.GB4169@localhost.localdomain>
Date:	Wed, 29 Jul 2009 21:06:39 -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 Thu, Jul 30, 2009 at 06:38:32AM +0800, Herbert Xu wrote:
> On Wed, Jul 29, 2009 at 03:43:00PM -0400, Neil Horman wrote:
> > 
> > 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.
> 
> When netpoll_poll calls dev->poll which then takes the TX lock 
> you have no protection whatsoever.  Some drivers do this in order
> to ensure that TX descriptor cleanups do not race against the
> transmitting side.  See tg3 for an example.

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.
Neil

> 
> Cheers,
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> Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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