Error APT: E: Unable to lock the download directory

Aquí otro tip rápido para nuestras cajas con Linux, si al hacer un:

$ apt-get upgrade

Nos devuelve un error algo así:

Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
  bind9 bind9-host dnsutils libbind9-30 libdns35 libisc35 libisccc30
  libisccfg30 liblwres30 linux-generic linux-headers-generic
  linux-image-generic linux-restricted-modules-generic netatalk
The following packages will be upgraded:
  libmysqlclient15-dev libmysqlclient15off mysql-client mysql-client-5.0
  mysql-common mysql-server mysql-server-5.0 php5-imap
8 upgraded, 0 newly installed, 0 to remove and 14 not upgraded.
3 not fully installed or removed.
E: Could not get lock /var/cache/apt/archives/lock - open (11 Resource temporarily unavailable)
E: Unable to lock the download directory

Lo único que debémos hacer como root es:

$ pkill apt