> > Now I reproduced the bug even with 4.7-rc1 so it is older than 4.7. Will 
> > test further.
> 
> It gets stranger and stranger - my old 4.7 image worked fine, freshly 
> compiled 4.7 exhibits the same problem.
> 
> Toolchain has not changed, that I know for sure.
> 
> What may have changed is kernel .config. My old conf was with whatever I 
> had during 4.7. Then I upgraded to 4.8-rc3 and then 4.8 and selected 
> values for "make oldconfig" new entries. Then went back to 4.7-rc1 and 
> then to 4.7 with this config, answering quiestion about new options when 
> any appeared. Diff is not available since I do not have the old configs 
> archived.

I did some more digging. Found an older configuration that is working 
and recreated a newer one that is bad, for the same 4.7 kernel. This is 
reproducible now, from "make clean" state.

Working config from 4.7-rc4 attached as config-4.7, broken config from 
4.7 attached as config-4.7-bad.

Will try to bisect the configs as time permits. But looking at the 
stack traces, the issue is probably timing related, when ip and dhclient 
do something with the same lock. seq_read that outputs stats could be 
reading /proc/net/dev that reads counters from each interface.

ifupdown seems to use the following for dhcp interfaces:
  up
    [[/bin/ip link set dev %iface% address %hwaddress%]]
    /sbin/dhclient -v -pf /run/dhclient.%iface%.pid -lf /var/lib/dhcp/dhclient.%iface%.leases -I -df /var/lib/dhcp/dhclient6.%iface%.leases %iface% \
...

so ip link is setting link up, this creates some work for the 
background, and the dhclient goes adn reads /proc/net/dev, and lockup is 
suspected but not proven?

I started a loop for test, doing cat /proc/net/dev in a loop and at the 
same link link up and down from console, but up and down is slow process 
and the loop did not seem to trigger the warning over night, so it was 
not so simple.


> > > [   83.716570] BUG: spinlock lockup suspected on CPU#0, dhclient/1014
> > > [   83.797819]  lock: 0xfff000123c8e4a08, .magic: dead4ead, .owner: ip/1001, .owner_cpu: 1
> > > [   83.903130] CPU: 0 PID: 1014 Comm: dhclient Not tainted 4.8.0 #4
> > > [   83.982129] Call Trace:
> > > [   84.014160]  [00000000004b7220] spin_dump+0x60/0xa0
> > > [   84.078203]  [00000000004b73a0] do_raw_spin_lock+0xa0/0x120
> > > [   84.106344] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
> > > [   84.107193] ip (1001) used greatest stack depth: 2168 bytes left
> > > [   84.306955]  [000000000092c0d0] _raw_spin_lock_bh+0x30/0x40
> > > [   84.380188]  [00000000100822cc] tg3_get_stats64+0xc/0x80 [tg3]
> > > [   84.456885]  [00000000007fac8c] dev_get_stats+0x2c/0xc0
> > > [   84.525506]  [000000000081a4e8] dev_seq_printf_stats+0x8/0xe0
> > > [   84.600986]  [000000000081a5e4] dev_seq_show+0x24/0x40
> > > [   84.668467]  [00000000005cb6c4] seq_read+0x2c4/0x440
> > > [   84.733656]  [000000000060b97c] proc_reg_read+0x3c/0x80
> > > [   84.802282]  [00000000005a219c] __vfs_read+0x1c/0x140
> > > [   84.868613]  [00000000005a2310] vfs_read+0x50/0x100
> > > [   84.932662]  [00000000005a265c] SyS_read+0x3c/0xa0
> > > [   84.995573]  [00000000004061d4] linux_sparc_syscall32+0x34/0x60
> > > [   85.073748] * CPU[  0]: TSTATE[00000044f0001a22] TPC[00000000f79a16b0] TNPC[00000000f79a16b4] TASK[dhclient:1014]
> > > [   85.208732]              TPC[f79a16b0] O7[f79405c8] I7[0] RPC[0]
> > > [   85.287633]   CPU[  1]: TSTATE[0000004480001605] TPC[00000000004b26f0] TNPC[00000000004d0b0c] TASK[swapper/1:0]
> > > [   85.420338]              TPC[trace_hardirqs_off+0x10/0x20] O7[rcu_idle_enter+0x64/0xa0] I7[cpu_startup_entry+0x1b0/0x240] RPC[rest_init+0x178/0x1a0]
> > > [   85.664600] tg3 0000:00:02.0 eth0: Link is up at 100 Mbps, full duplex
> > > [   85.750515] tg3 0000:00:02.0 eth0: Flow control is off for TX and off for RX
> > > [   85.843994] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready

-- 
Meelis Roos (mroos@linux.ee)