unregister_netdevice(): negative refcnt, suggest patch against 2.6.11

unregister_netdevice(): negative refcnt, suggest patch against 2.6.11

[Felix Palmen]

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Hi lkml-members,

I recently had a problem with appletalk. After starting atalkd on a TAP
interface and stopping it later, unregister_netdevice() just stated
| unregister_netdevice: waiting for tap0 to become free. Usage count = -1

So I assume there is a problem in the appletalk code, but I didn't try
reproducing that on other systems so far.

I changed my kernel to "correct" a negative refcnt to 0 and that kind
of fixes the problem. I'm not sure whether this could break anything,
but certainly waiting for a device to become free is no use when there
is a negative number of users, so I think it would be better to allow
the system to shut down cleanly in this case.

Here's m suggestion:

#v+
--- linux-2.6.11-orig/net/core/dev.c 2005-03-02 08:38:09.000000000 +0100
+++ linux-2.6.11/net/core/dev.c    2005-04-09 16:44:42.000000000 +0200
@@ -2876,7 +2876,7 @@
	unsigned long rebroadcast_time, warning_time;

	rebroadcast_time = warning_time = jiffies;
-	while (atomic_read(&dev->refcnt) != 0) {
+	while (atomic_read(&dev->refcnt) > 0) {
		if (time_after(jiffies, rebroadcast_time + 1 * HZ)) {
			rtnl_shlock();

@@ -2910,6 +2910,13 @@
			warning_time = jiffies;
		}
	}
+	if (atomic_read(&dev->refcnt) != 0) {
+		printk(KERN_ERR "unregister_netdevice: "
+			"%s has negative refcnt (%d). "
+			"This should never happen! Setting refcnt to 0.\n",
+			dev->name, atomic_read(&dev->refcnt));
+		atomic_set(&dev->refcnt, 0);
+	}
 }

 /* The sequence is:
#v-

Greets, Felix

PS: Please note I'm not subscribed to lkml and CC me in replies, thanks.

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Re: unregister_netdevice(): negative refcnt, suggest patch against 2.6.11

[Felix Palmen]

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* Felix M. Palmen <fmp@palmen.homeip.net> [20050410 11:16]:
> So I assume there is a problem in the appletalk code, but I didn't try
> reproducing that on other systems so far.

I've now tested this issue on a vanilla 2.6.11.7 kernel. I only applied
my own patch from the previous post so that I am able to shut down the
computer cleanly. So I did the following:

- boot 2.6.11.7
- create a tap device with 'openvpn --mktun --dev tap0'
- create /etc/netatalk/atalkd.conf with a single line 'tap0'
- launch atalkd, wait.
- stop atalkd.
- destroy tap device with 'openvpn --rmtun --dev tap0'

refcnt was -256, very strange.

Would you consider my patch harmful? I intend using it for now, because
otherwise, my system won't shut down cleanly any more...

Greets, Felix

PS: Please CC replies to me.

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