Comment # 4 on bug 1191519 from Jiri Slaby

(In reply to Linus Kardell from comment #2)
> Freezing user space processes ... 
> bpfilter: read fail -512
> 
> Freezing of tasks failed after 20.003 seconds (1 tasks refusing to freeze, wq_busy=0):
> task:xtables-legacy- state:D stack:0     pid:9782  ppid:9781   flags:0x00004006
> Call Trace:
>  <TASK>
>  __schedule+0x360/0x1350
>  schedule+0x5a/0xd0
>  shutdown_umh+0x7d/0xcc [bpfilter edf9c52c8eee3a505298e3d0d1a17423bd74c763]
>  bpfilter_send_req.cold+0x14/0x2f [bpfilter edf9c52c8eee3a505298e3d0d1a17423bd74c763]
>  bpfilter_process_sockopt+0x57/0x80 [bpfilter edf9c52c8eee3a505298e3d0d1a17423bd74c763]
>  bpfilter_mbox_request+0x84/0x130
>  ip_getsockopt+0x9b/0xf0
>  __sys_getsockopt+0xad/0x1b0
>  __x64_sys_getsockopt+0x1b/0x30
>  do_syscall_64+0x5b/0x80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f828a7c33fe
> RSP: 002b:00007ffec6528e18 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f828a7c33fe
> RDX: 0000000000000040 RSI: 0000000000000000 RDI: 0000000000000004
> RBP: 0000000000000004 R08: 00007ffec6528e50 R09: 0000000000004000
> R10: 00007ffec6528e60 R11: 0000000000000246 R12: 0000000000000000
> R13: 000055c3d3350f8e R14: 00007ffec6528e60 R15: 00007ffec652ae86
>  </TASK>

> Looks like task xtables-legacy blocks suspend until it times out and fails
> after 20 seconds.

It's likely xtables-legacy-multi doing getsockopt. And bpfilter trying to wait
for umh process group. But that doesn't exit during suspend, obviously.