Ubuntu – Network manager goes bad after hot unplugging an external usb wifi adapter

network-interfaceUbuntuusb

While my Lubuntu 18.04 is connected to a wifi network via an external wifi adapter, I hot unplug the external wifi adapter.

Symptoms of the problem:

  1. When I plug the external wifi adapter back to my computer, no wifi network is detected, even though both networking and wifi are enbaled.

  2. The top process shown in the output of top is:

     PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 1068 root      20   0 1029552   8060   4864 R  85.7  0.1  29:25.70 NetworkManager

    Trying to kill it by sudo kill 1068 or sudo service NetworkManager restart/stop or sudo systemctl restart/stop NetworkManager will result in hanging forever

    Trying to run any sudo command e.g. sudo pm-suspend will result in the same forever
    hanging

  3. The usb port on my laptop where the external wifi adapter was
    originally plugged in has also stop working for other external
    devices, unless I reboot my Lubuntu.

How can I solve the problem? Preferably not rebooting my Lubuntu.

How can I avoid the problem in the future?

Thanks.

Log:

This is part of /var/log/syslog around the time when the problem happened. (I unplugged a keyword to free up a usb port for a flash drive, and then unplugged the external wifi adapter, to free up a usb port for a printer, and then plugged it back. )

This is from journalctl --since "2 days ago".

USB subsystems:

I also followed https://www.linux.org/threads/resetting-the-usb-subsystem.10404/ suggested by Network manager goes bad after hot unplugging an external usb wifi adapter

$ lsusb 
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 048: ID 413c:2113 Dell Computer Corp. 
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 009: ID 7392:7811 Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 002: ID 08ff:2810 AuthenTec, Inc. AES2810
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

$ lspci | grep USB
00:1a.0 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 (rev 03)
00:1a.1 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5 (rev 03)
00:1a.2 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #6 (rev 03)
00:1a.7 USB controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2 (rev 03)
00:1d.0 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 (rev 03)
00:1d.1 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 (rev 03)
00:1d.2 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 (rev 03)
00:1d.7 USB controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 (rev 03)

But because sudo commands seem not working, the following sudo command doesn't work (also see https://askubuntu.com/questions/1153207/lsusb-i-get-always-couldnt-open-device-is-this-normal):

$ lsusb -v -s 1:9 | grep iSerial
Couldn't open device, some information will be missing
  iSerial                 3 

$ sudo lsusb -v -s 1:9 | grep iSerial


$ lsusb -v -s 1:9

Couldn't open device, some information will be missing

Bus 001 Device 009: ID 7392:7811 Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x7392 Edimax Technology Co., Ltd
  idProduct          0x7811 EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
  bcdDevice            2.00
  iManufacturer           1 
  iProduct                2 
  iSerial                 3 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           46
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           4
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x84  EP 4 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               1

Best Answer

As Vasconcelos writes above, "sudo service NetworkManager restart" (or the similar "sudo systemctl restart NetworkManager") is the way to fix your immediate problem.

The reason using "kill" to stop NetworkManager is a bad idea is that NM is the process that manages the network. It is not going to gracefully terminate until every process that uses the network also terminate -- which include some processes inside the kernel. (Well, "kill -9" might stop NM, but that will leave a nasty mess. Sorta like stopping your car at a stop light by ripping out all of the spark plug wires.)

You have two solutions here:

  1. Find the network interface with "ip a", bring it down with "sudo link set $NAME_OF_PORT down", then remove it. That way NM is not blocking the USB port, & you ought to be able to plug another USB device into it.

  2. Stop NM with "sudo service NetworkManager stop" (or "sudo systemctl stop NetworkManager"), remove the wireless WiFi adapter, then restart NetworkManager. A little more drastic, but it ought to work.

If neither works, query the manufacturer. They ought to know how a user can gracefully remove their product from a Linux system, especially if it requires some non-standard steps. (Their website might even have the answer, so you don't have to go thru call center Hell to find someone who knows where to find the answer.)

ADDENDUM: When I mentioned "journalctl", I meant that will be the tool you will need to use to further investigate this problem. Looking at the snippet you attached, I see all sorts of possibilities -- but none I'm certain is the problem.

I'll suggest you try the following steps:

First, powercycle the system so it is back in a known stable state. Then use sudo to execute a command, say sudo nmcli con show If the command completes & does not hang, continue on. If not, then there is something going on with this computer I can't diagnose from here. (You might have to reinstall the OS; it will be the simplest solution.)

Next, execute sudo nmcli radio all This ought to confirm you have WiFi working.

At this point, whenever you remove that USB wireless device, execute sudo nmcli radio wifi off first, then remove the device. This will tell NetworkManager to stop looking for it -- which appears to be the cause of your problem. Then, after you reconnect the device, execute sudo nmcli radio wifi on, which will tell NetworkManager to start looking for it.

Here I have to stop. If this still doesn't help you with your problem, I'm out of ideas. I'll admit I'm still learning how to work with Network Manager; I've been able to avoid using it until now, but now have been forced to learn it.

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