netdev - Re: netpoll + xmit

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Message-ID: <20090729231735.GB14066@gondor.apana.org.au>
Date:	Thu, 30 Jul 2009 07:17:35 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Neil Horman <nhorman@...driver.com>
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 Wed, Jul 29, 2009 at 07:15:17PM -0400, Neil Horman wrote:
>
> Not quite.  I agree private locking in a driver is a pain when you consider
> netpoll clients, its not the tx/tx recursion you need to worry about though, its
> shared locking between the tx and rx path that you need to be worried about.
> We should be protected against deadlock on the _xmit_lock from what we discussed
> above, but if you take a lock in the driver, then call printk, its possible that
> you'll go down the ->poll routine path in the driver.  If there you try to take
> the same private lock, the result is then deadlock.

xmit_lock suffers from exactly the same problem in ->poll.

> I was thinking that perhaps what we should do is simply not call netpoll_poll
> from within netpoll_send_skb.  That breaks the only spot that I see in which we
> call a receive code from within the tx path, breaking the deadlock possibilty.
> Perhaps instead we can call netif_rx_schedule on the network interfaces napi
> struct.  We already queue the frames and set a timer to try sending again later.
> By calling netif_rx_schedule, we move the receive work to the net_rx_action
> softirq (where it really should be).
> 
> Thoughts?

Alternatively we can modify the drivers to use try lock or other
mechanisms that do not result in a dead-lock.

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