INFO: task snmpd:398 blocked for more than 120 seconds.

[Joakim Tjernlund]

I can't make out what is causing this hang every now an then:

INFO: task snmpd:398 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
snmpd         D 0fad1760     0   398      1 0x00000000
Call Trace:
[c7b5fc70] [c7a49a80] 0xc7a49a80 (unreliable)
[c7b5fd30] [c0008608] __switch_to+0x4c/0x6c
[c7b5fd40] [c028fed4] schedule+0x184/0x310
[c7b5fd70] [c0290e5c] __mutex_lock_slowpath+0xa4/0x150
[c7b5fdb0] [c0290c50] mutex_lock+0x44/0x48
[c7b5fdc0] [c01eb5b0] rtnl_lock+0x18/0x28
[c7b5fdd0] [c01df6c4] dev_ioctl+0x2b4/0x764
[c7b5fe60] [c01cafc8] sock_ioctl+0x18c/0x2a0
[c7b5fe80] [c009a16c] vfs_ioctl+0xcc/0xe0
[c7b5fea0] [c009a998] do_vfs_ioctl+0xc4/0x79c
[c7b5ff10] [c009b0b0] sys_ioctl+0x40/0x74
[c7b5ff40] [c00117c4] ret_from_syscall+0x0/0x38

The dev_ioctl is in SIOCGIFCONF:
int dev_ioctl(struct net *net, unsigned int cmd, void __user *arg)
{
	struct ifreq ifr;
	int ret;
	char *colon;

	/* One special case: SIOCGIFCONF takes ifconf argument
	   and requires shared lock, because it sleeps writing
	   to user space.
	 */

	if (cmd == SIOCGIFCONF) {
		rtnl_lock();
		ret = dev_ifconf(net, (char __user *) arg);
		rtnl_unlock();
		return ret;
	}

Any ideas? Kernel is 2.6.35