Re: 2.6.18-rc5 with GRE, iptables and Speedtouch ADSL, PPP over ATM

[Herbert Xu <herbert@gondor.apana.org.au>]

[NET]: Drop tx lock in dev_watchdog_up

On Sat, Sep 02, 2006 at 08:39:28PM +0000, Krzysztof Halasa wrote:
> 
> =======================================================
> [ INFO: possible circular locking dependency detected ]
> -------------------------------------------------------
> swapper/0 is trying to acquire lock:
>  (&dev->queue_lock){-+..}, at: [<c02c8c46>] dev_queue_xmit+0x56/0x290
> 
> but task is already holding lock:
>  (&dev->_xmit_lock){-+..}, at: [<c02c8e14>] dev_queue_xmit+0x224/0x290
> 
> which lock already depends on the new lock.

This turns out to be a genuine bug.  The queue lock and xmit lock are
intentionally taken out of order.  Two things are supposed to prevent
dead-locks from occuring:

1) When we hold the queue_lock we're supposed to only do try_lock on the
tx_lock.

2) We always drop the queue_lock after taking the tx_lock and before doing
anything else.

> 
> the existing dependency chain (in reverse order) is:
> 
> -> #1 (&dev->_xmit_lock){-+..}:
>        [<c012e7b6>] lock_acquire+0x76/0xa0
>        [<c0336241>] _spin_lock_bh+0x31/0x40
>        [<c02d25a9>] dev_activate+0x69/0x120

This path obviously breaks assumption 1) and therefore can lead to ABBA
dead-locks.

I've looked at the history and there seems to be no reason for the lock
to be held at all in dev_watchdog_up.  The lock appeared in day one and
even there it was unnecessary.  In fact, people added __dev_watchdog_up
precisely in order to get around the tx lock there.

The function dev_watchdog_up is already serialised by rtnl_lock since
its only caller dev_activate is always called under it.

So here is a simple patch to remove the tx lock from dev_watchdog_up.
In 2.6.19 we can eliminate the unnecessary __dev_watchdog_up and
replace it with dev_watchdog_up.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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--
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 0834c2e..6f91518 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -238,9 +238,7 @@ void __netdev_watchdog_up(struct net_dev
 
 static void dev_watchdog_up(struct net_device *dev)
 {
-	netif_tx_lock_bh(dev);
 	__netdev_watchdog_up(dev);
-	netif_tx_unlock_bh(dev);
 }
 
 static void dev_watchdog_down(struct net_device *dev)

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