This bug was fixed in the package linux-aws - 5.4.0-1021.21
---------------
linux-aws (5.4.0-1021.21) focal; urgency=medium
* focal/linux-aws: 5.4.0-1021.21 -proposed tracker (LP: #1888811)
* xen-netfront: potential deadlock in xennet_remove() (LP: #1888510)
- SAUCE: xen-netfront: fix potential deadlock in xennet_remove()
-- Stefan Bader <stefan.ba...@canonical.com> Fri, 24 Jul 2020 11:24:21
+0200
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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 Released
Status in linux-aws source package in Focal:
Fix Released
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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