Re: [2.6.17-git22] lock debugging output

[Arjan van de Ven <arjan@infradead.org>]

From: Arjan van de Ven <arjan@linux.intel.com>

On Tue, 2006-07-04 at 20:13 +0200, Alessandro Suardi wrote:
> Hoping gmail doesn't mess it too badly...
> 
> eth0: tg3 (BCM5751 Gbit Ethernet)
> eth1: ipw2200 (Intel PRO/Wireless 2200BG)
> 
> Sequence:
>  1. boot with eth0 disconnected (eth1 doesn't come up on boot)
>  2. ifup eth1, bring wpa-supplicant up
>  3. run 'dig' ---> <lock debug info gets printed on console>


this appears to be a real deadlock:

the SO_BINDTODEVICE ioctl calls sk_dst_reset(), which looks like this:
static inline void
sk_dst_reset(struct sock *sk)
{
        write_lock(&sk->sk_dst_lock);
        __sk_dst_reset(sk);
        write_unlock(&sk->sk_dst_lock);
}

now... ipv6 does this in softirq context:
  [<c028cf72>] sk_dst_check+0x1b/0xe6
  [<f8ce1305>] ip6_dst_lookup+0x31/0x16d [ipv6]
  [<f8cf3338>] icmpv6_send+0x332/0x549 [ipv6]
  [<f8cf09a1>] udpv6_rcv+0x4ab/0x4d6 [ipv6]
  [<f8ce2900>] ip6_input+0x19c/0x228 [ipv6]
  [<f8ce2d61>] ipv6_rcv+0x188/0x1b7 [ipv6]
  [<c02925b7>] netif_receive_skb+0x18d/0x1d8
  [<c0293d6a>] process_backlog+0x80/0xf9
  [<c0293f43>] net_rx_action+0x80/0x174
  [<c01162fd>] __do_softirq+0x46/0x9c
  [<c01040e6>] do_softirq+0x4d/0xac

where sk_dst_check() takes the same lock for read.

that looks like a real deadlock to me... 
the most obvious low impact solution is to make sk_dst_reset use an
irqsave variant; patch for that is attached below. I'll leave it to the
networking people to say if that's the real right approach

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

---
 include/net/sock.h |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6.17-mm6/include/net/sock.h
===================================================================
--- linux-2.6.17-mm6.orig/include/net/sock.h
+++ linux-2.6.17-mm6/include/net/sock.h
@@ -1025,9 +1025,10 @@ __sk_dst_reset(struct sock *sk)
 static inline void
 sk_dst_reset(struct sock *sk)
 {
-	write_lock(&sk->sk_dst_lock);
+	unsigned long flags;
+	write_lock_irqsave(&sk->sk_dst_lock, flags);
 	__sk_dst_reset(sk);
-	write_unlock(&sk->sk_dst_lock);
+	write_unlock_irqrestore(&sk->sk_dst_lock, flags);
 }
 
 extern struct dst_entry *__sk_dst_check(struct sock *sk, u32 cookie);


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