netdev - Re: netpoll + xmit

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Message-ID: <20090731130243.GA31058@gondor.apana.org.au>
Date:	Fri, 31 Jul 2009 21:02:43 +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 Fri, Jul 31, 2009 at 08:56:54AM -0400, Neil Horman wrote:
>
> > 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.

No that can't happen because BH is disabled in the xmit function.

This problem is specific to netpoll because it does things that
normally can't happen with BH off.

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