Subject: macvtap bug: using smp_processor_id() in preemptible code

From: Thomas Huth <[email-masked]>
Date: Wed, 7 Aug 2013 16:43:19 +0200

Hi,

I am using macvtap (via KVM/virsh) on a s390 box. With the latest
kernel source from linux-next master branch, I suddenly get the
following error messages in the dmesg output:

BUG: using smp_processor_id() in preemptible [00000000] code: vhost-45891/45892
caller is macvtap_do_read+0x45c/0x600 [macvtap]
CPU: 1 PID: 45892 Comm: vhost-45891 Not tainted 3.11.0-bisecttest #13
       000000010cab3a00 000000010cab3a10 0000000000000002 0000000000000000 
       000000010cab3aa0 000000010cab3a18 000000010cab3a18 00000000001127b4 
       0000000000000000 0000000000000001 0000000000000000 000000010000000b 
       0000000000000060 000003fe00000008 0000000000000000 000000010cab3a70 
       00000000006ea2f0 00000000001127b4 000000010cab3a00 000000010cab3a50 
Call Trace:
([<00000000001126ee>] show_trace+0x126/0x144)
 [<00000000001127d2>] show_stack+0xc6/0xd4
 [<000000000068bcec>] dump_stack+0x74/0xd8
 [<0000000000481066>] debug_smp_processor_id+0xf6/0x114
 [<000003ff802e9a18>] macvtap_do_read+0x45c/0x600 [macvtap]
 [<000003ff802e9c1c>] macvtap_recvmsg+0x60/0x88 [macvtap]
 [<000003ff80318c5e>] handle_rx+0x5b2/0x800 [vhost_net]
 [<000003ff8028f77c>] vhost_worker+0x15c/0x1c4 [vhost]
 [<000000000015f3ac>] kthread+0xd8/0xe4
 [<00000000006934a6>] kernel_thread_starter+0x6/0xc
 [<00000000006934a0>] kernel_thread_starter+0x0/0xc
2 locks held by vhost-45891/45892:
 #0:  (&vq->mutex){+.+...}, at: [<000003ff80318718>] handle_rx+0x6c/0x800 [vhost_net]
 #1:  (rcu_read_lock){.+.+..}, at: [<000003ff802e98fe>] macvtap_do_read+0x342/0x600 [macvtap]

Apart from these "annoying" error messages (they are repeated
continually while the KVM guest is running / macvtap is in use),
everything still seems to work fine, though.

Since everything was still working fine in v3.10, I did some bisecting
and it seems like this commit introduced this problem:

  commit: ac4e4af1e59e16a018527ffa58d9d3f30bb96ca9
  Subject: macvtap: Consistently use rcu functions

I am not sure how to proceed here, this is my first bug report in this
area, so advice is welcome...

 Regards,
  Thomas Huth