[Qemu-devel] [Bug 1823458] Re: race condition between vhost_net_stop and CHR_EVENT_CLOSED on shutdown crashes qemu

[Dan Streetman]

test builds in https://launchpad.net/~ddstreet/+archive/ubuntu/lp1823458

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Title:
  race condition between vhost_net_stop and CHR_EVENT_CLOSED on shutdown
  crashes qemu

Status in QEMU:
  In Progress
Status in qemu package in Ubuntu:
  In Progress
Status in qemu source package in Trusty:
  In Progress
Status in qemu source package in Xenial:
  In Progress
Status in qemu source package in Bionic:
  In Progress
Status in qemu source package in Cosmic:
  In Progress
Status in qemu source package in Disco:
  In Progress

Bug description:
  [impact]

  on shutdown of a guest, there is a race condition that results in qemu
  crashing instead of normally shutting down.  The bt looks similar to
  this (depending on the specific version of qemu, of course; this is
  taken from 2.5 version of qemu):

  (gdb) bt
  #0  __GI___pthread_mutex_lock (mutex=0x0) at ../nptl/pthread_mutex_lock.c:66
  #1  0x00005636c0bc4389 in qemu_mutex_lock (address@hidden) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/util/qemu-thread-posix.c:73
  #2  0x00005636c0988130 in qemu_chr_fe_write_all (address@hidden, 
address@hidden "\v", address@hidden) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/qemu-char.c:205
  #3  0x00005636c08f3483 in vhost_user_write (address@hidden, address@hidden, 
address@hidden, dev=0x5636c1bf6b70, dev=0x5636c1bf6b70)
      at /build/qemu-7I4i1R/qemu-2.5+dfsg/hw/virtio/vhost-user.c:195
  #4  0x00005636c08f411c in vhost_user_get_vring_base (dev=0x5636c1bf6b70, 
ring=0x7ffe65c087e0) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/hw/virtio/vhost-user.c:364
  #5  0x00005636c08efff0 in vhost_virtqueue_stop (address@hidden, 
address@hidden, vq=0x5636c1bf6d00, idx=1) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/hw/virtio/vhost.c:895
  #6  0x00005636c08f2944 in vhost_dev_stop (address@hidden, address@hidden) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/hw/virtio/vhost.c:1262
  #7  0x00005636c08db2a8 in vhost_net_stop_one (net=0x5636c1bf6b70, 
address@hidden) at /build/qemu-7I4i1R/qemu-2.5+dfsg/hw/net/vhost_net.c:293
  #8  0x00005636c08dbe5b in vhost_net_stop (address@hidden, ncs=0x5636c209d110, 
address@hidden) at /build/qemu-7I4i1R/qemu-2.5+dfsg/hw/net/vhost_net.c:371
  #9  0x00005636c08d7745 in virtio_net_vhost_status (status=7 '\a', 
n=0x5636c2853338) at /build/qemu-7I4i1R/qemu-2.5+dfsg/hw/net/virtio-net.c:150
  #10 virtio_net_set_status (vdev=<optimized out>, status=<optimized out>) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/hw/net/virtio-net.c:162
  #11 0x00005636c08ec42c in virtio_set_status (vdev=0x5636c2853338, 
val=<optimized out>) at /build/qemu-7I4i1R/qemu-2.5+dfsg/hw/virtio/virtio.c:624
  #12 0x00005636c098fed2 in vm_state_notify (address@hidden, address@hidden) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/vl.c:1605
  #13 0x00005636c089172a in do_vm_stop (state=RUN_STATE_SHUTDOWN) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/cpus.c:724
  #14 vm_stop (state=RUN_STATE_SHUTDOWN) at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/cpus.c:1407
  #15 0x00005636c085d240 in main_loop_should_exit () at 
/build/qemu-7I4i1R/qemu-2.5+dfsg/vl.c:1883
  #16 main_loop () at /build/qemu-7I4i1R/qemu-2.5+dfsg/vl.c:1931
  #17 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) 
at /build/qemu-7I4i1R/qemu-2.5+dfsg/vl.c:4683

  [test case]

  unfortunately since this is a race condition, it's very hard to
  arbitrarily reproduce; it depends very much on the overall
  configuration of the guest as well as how exactly it's shut down -
  specifically, its vhost user net must be closed from the host side at
  a specific time during qemu shutdown.

  I have someone with such a setup who has reported to me their setup is
  able to reproduce this reliably, but the config is too complex for me
  to reproduce so I have relied on their reproduction and testing to
  debug and craft the patch for this.

  [regression potential]

  the change adds flags to prevent repeated calls to both
  vhost_net_stop() and vhost_net_cleanup() (really, prevents repeated
  calls to vhost_dev_cleanup(), but vhost_net_cleanup() does nothing
  else).  Any regression would be seen when stopping and/or cleaning up
  a vhost net.  Regressions might include failure to hot-remove a vhost
  net from a guest, or failure to cleanup (i.e. mem leak), or crashes
  during cleanup or stopping a vhost net.

  However, the flags are very unintrusive, and only in the shutdown path
  (of a vhost_dev or vhost_net), and are unlikely to cause any
  regressions.

  [other info]

  this was originally seen in the 2.5 version of qemu - specifically,
  the UCA version in trusty-mitaka (which uses the xenial qemu
  codebase).  However, this appears to still apply upstream, and I am
  sending a patch to the qemu list to patch upstream as well.

  The specific race condition for this (in the qemu 2.5 code version)
  is:

  as shown in above bt, thread A starts shutting down qemu, e.g.:

  vm_stop->do_vm_stop->vm_state_notify
    virtio_set_status
      virtio_net_set_status
        virtio_net_vhost_status

  in this function, code gets to an if-else check for
  (!n->vhost_started), which is false (i.e. vhost_started is true) and
  enters the else code block, which calls vhost_net_stop() and then sets
  n->vhost_started to false.

  While thread A is inside vhost_net_stop(), thread B is triggered by
  the vhost net chr handler with a user event and calls:

  net_vhost_user_event
    qmp_set_link (from case CHR_EVENT_CLOSED)
      virtio_net_set_link_status (via ->link_status_changed)
        virtio_net_set_status
          virtio_net_vhost_status

  notice thread B has now reached the same function that thread A is in;
  since the checks in the function have not changed, thread B follows
  the same path that thread A followed, and enters vhost_net_stop().

  Since thread A has already shut down and cleaned up some of the
  internals, once thread B starts trying to also clean up things, it
  segfaults as the shown in the bt.

  Avoiding only this duplicate call to vhost_net_stop() is required, but
  not enough - let's continue to look at what thread B does after its
  call to qmp_set_link() returns:

  net_vhost_user_event
    vhost_user_stop
      vhost_net_cleanup
        vhost_dev_cleanup

  However, in main() qemu registers atexit(net_cleanup()), which does:
  net_cleanup
    qemu_del_nic (or qemu_del_net_client, depending on ->type)
      qemu_cleanup_net_client
        vhost_user_cleanup (via ->cleanup)
          vhost_net_cleanup
            vhost_dev_cleanup

  and the duplicate vhost_dev_cleanup fails assertions since things were
  already cleaned up.

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