** Changed in: linux-aws (Ubuntu)
       Status: New => Triaged

** Changed in: linux-aws (Ubuntu)
       Status: Triaged => Invalid

** Changed in: linux-aws (Ubuntu Bionic)
       Status: New => Incomplete

** Changed in: linux-aws (Ubuntu Focal)
       Status: New => Fix Committed

** Changed in: linux-aws (Ubuntu Focal)
   Importance: Undecided => High

** Changed in: linux-aws-5.3 (Ubuntu Bionic)
       Status: New => Fix Committed

** Changed in: linux-aws-5.3 (Ubuntu Bionic)
   Importance: Undecided => High

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You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux-aws in Ubuntu.
https://bugs.launchpad.net/bugs/1888510

Title:
  xen-netfront: potential deadlock in xennet_remove()

Status in linux-aws package in Ubuntu:
  Invalid
Status in linux-aws-5.3 package in Ubuntu:
  Invalid
Status in linux-aws source package in Bionic:
  Incomplete
Status in linux-aws-5.3 source package in Bionic:
  Fix Committed
Status in linux-aws source package in Focal:
  Fix Committed
Status in linux-aws-5.3 source package in Focal:
  Invalid

Bug description:
  [Impact]

  During our AWS testing we were experiencing deadlocks on hibernate across all 
Xen instance types.
  The trace was showing that the system was stuck in xennet_remove():

  [  358.109087] Freezing of tasks failed after 20.006 seconds (1 tasks 
refusing to freeze, wq_busy=0):
  [  358.115102] modprobe        D    0  4892   4833 0x00004004
  [  358.115104] Call Trace:
  [  358.115112]  __schedule+0x2a8/0x670
  [  358.115115]  schedule+0x33/0xa0
  [  358.115118]  xennet_remove+0x1f0/0x230 [xen_netfront]
  [  358.115121]  ? wait_woken+0x80/0x80
  [  358.115124]  xenbus_dev_remove+0x51/0xa0
  [  358.115126]  device_release_driver_internal+0xe0/0x1b0
  [  358.115127]  driver_detach+0x49/0x90
  [  358.115129]  bus_remove_driver+0x59/0xd0
  [  358.115131]  driver_unregister+0x2c/0x40
  [  358.115132]  xenbus_unregister_driver+0x12/0x20
  [  358.115134]  netif_exit+0x10/0x7aa [xen_netfront]
  [  358.115137]  __x64_sys_delete_module+0x146/0x290
  [  358.115140]  do_syscall_64+0x5a/0x130
  [  358.115142]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

  This prevented hibernation to complete.

  The reason of this problem is a race condition in xennet_remove(): the
  system is reading the current state of the bus, it's requesting to
  change the state to "Closing", and it's waiting for the state to be
  changed to "Closing". However, if the state becomes "Closed" between
  reading the state and requesting the state change, we are stuck
  forever, because the state will never change from "Closed" back to
  "Closing".

  [Test case]

  Create any Xen-based instance in AWS, hibernate/resume multiple times.
  Some times the system gets stuck (hung task timeout).

  [Fix]

  Prevent the deadlock by changing the wait condition to check also for
  state == Closed.

  [Regression potential]

  Minimal, this change affects only Xen, more exactly only the xen-
  netfront driver.

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