Subject: Re: WARNING: at include/net/sock.h:417 udp_lib_unhash

From: Eric Dumazet <[email-masked]>
Date: Thu, 02 Jul 2009 08:10:14 +0200

Tantilov, Emil S a écrit :
> I see the following trace during netperf stress mixed UDP/TCP IPv4/6 traffic. This is on recent pulls from net-2.6 and net-next.
> 
> [45197.989163] ------------[ cut here ]------------
> [45197.994309] WARNING: at include/net/sock.h:417 udp_lib_unhash+0x81/0xab()
> [45197.994311] Hardware name: X7DA8
> [45197.994314] Modules linked in: e1000 [last unloaded: e1000]
> [45197.994326] Pid: 7110, comm: netserver Tainted: G        W  2.6.31-rc1-net-next-e1000-06250902 #8
> [45197.994331] Call Trace:
> [45197.994336]  [<ffffffff8135e0dc>] ? udp_lib_unhash+0x81/0xab
> [45197.994344]  [<ffffffff8103cac9>] warn_slowpath_common+0x77/0x8f
> [45197.994349]  [<ffffffff8103caf0>] warn_slowpath_null+0xf/0x11
> [45197.994352]  [<ffffffff8135e0dc>] udp_lib_unhash+0x81/0xab
> [45197.994357]  [<ffffffff81301acb>] sk_common_release+0x2f/0xb4
> [45197.994364]  [<ffffffff813a0256>] udp_lib_close+0x9/0xb
> [45197.994369]  [<ffffffff81364259>] inet_release+0x58/0x5f
> [45197.994374]  [<ffffffff8138c8bd>] inet6_release+0x30/0x35
> [45197.994383]  [<ffffffff812ff273>] sock_release+0x1a/0x6c
> [45197.994386]  [<ffffffff812ff763>] sock_close+0x22/0x26
> [45197.994392]  [<ffffffff810c69a0>] __fput+0xf0/0x18c
> [45197.994395]  [<ffffffff810c6d00>] fput+0x15/0x19
> [45197.994399]  [<ffffffff810c3c3e>] filp_close+0x5c/0x67
> [45197.994404]  [<ffffffff810c3cc4>] sys_close+0x7b/0xb6
> [45197.994412]  [<ffffffff8100baeb>] system_call_fastpath+0x16/0x1b
> [45197.994418] ---[ end trace 5acab6fc0afdaaa3 ]---
> 
> Emil--

Thanks for this report Emil.

I could not find a recent change in this area in last kernels.
If struct sk is hashed (sk_hashed() true), then sk_refcnt was incremented
in sk_nulls_add_node_rcu(), thus its value should be >= 2.

Maybe we have a missing memory barrier somewhere or a list corruption.

1) Could you try CONFIG_DEBUG_LIST=y ?

2) Could you give model of cpu, since it reminds me the ongoing discussion raised by Jiri Olsa.

CPU1 does an atomic_inc(&sk->sk_refcnt)  : refcnt changes from 1 to 2
then CPU2 does an atomic_read(&sk->sk_refcnt) and reads 1 instead of 2

David, maybe this test is not safe and if we really want to do a check
we need to use a stronger atomic function.

If you can reproduce this problem easily could you try following patch ?

Thank you

diff --git a/include/net/sock.h b/include/net/sock.h
index 352f06b..96ab278 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -393,8 +393,9 @@ static __inline__ int sk_del_node_init(struct sock *sk)
 
 	if (rc) {
 		/* paranoid for a while -acme */
-		WARN_ON(atomic_read(&sk->sk_refcnt) == 1);
-		__sock_put(sk);
+		int res = atomic_dec_return(&sk->sk_refcnt);
+
+		WARN_ON(res <= 0);
 	}
 	return rc;
 }
@@ -413,9 +414,9 @@ static __inline__ int sk_nulls_del_node_init_rcu(struct sock *sk)
 	int rc = __sk_nulls_del_node_init_rcu(sk);
 
 	if (rc) {
-		/* paranoid for a while -acme */
-		WARN_ON(atomic_read(&sk->sk_refcnt) == 1);
-		__sock_put(sk);
+		int res = atomic_dec_return(&sk->sk_refcnt);
+
+		WARN_ON(res <= 0);
 	}
 	return rc;
 }