From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoffer Dall <christoffer.dall@linaro.org>
Subject: [BUG] tuntap: using smp_processor_id() in preemptible from xdp flush
Date: Tue, 13 Feb 2018 14:21:47 +0100
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Cc: davem@davemloft.net, Marc Zyngier <marc.zyngier@arm.com>,
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Hi Jason,

With v4.16-rc1 I see a low of these when running my KVM/ARM test loop:

  BUG: using smp_processor_id() in preemptible [00000000] code: vhost-2877/2900
  caller is debug_smp_processor_id+0x1c/0x28
  CPU: 0 PID: 2900 Comm: vhost-2877 Not tainted 4.16.0-rc1 #1333
  Hardware name: APM X-Gene Mustang board (DT)
  Call trace:
   dump_backtrace+0x0/0x180
   show_stack+0x24/0x30
   dump_stack+0x8c/0xac
   check_preemption_disabled+0xf8/0x100
   debug_smp_processor_id+0x1c/0x28
   xdp_do_flush_map+0x24/0x48
   tun_sendmsg+0x90/0xa0
   handle_tx+0x254/0x548
   handle_tx_kick+0x20/0x30
   vhost_worker+0xc0/0x158
   kthread+0x104/0x130
   ret_from_fork+0x10/0x1c

I confirmed that reverting
  762c330d670e, "tuntap: add missing xdp flush", 2018-02-07
solves the problem for me.

I'm not at all familiar with this part of the kernel and not sure what
the proper fix is.  I'd be grateful if you could take a look and I'm
happy to help test etc.

Thanks,
-Christoffer