Killing servo processes doesn't always work

[jdm]

From http://build.servo.org/builders/linux-rel/builds/367:

pkill -x servo
 in dir /home/servo/buildbot/slave/linux-rel/build (timeout 1200 secs)
 watching logfiles {}
 argv: ['pkill', '-x', 'servo']
 environment:
  HOME=/home/servo
  PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
  PWD=/home/servo/buildbot/slave/linux-rel/build
  TERM=linux
  UPSTART_INSTANCE=
  UPSTART_JOB=buildbot-slave
 using PTY: False

command timed out: 1200 seconds without output running ['pkill', '-x', 'servo'], attempting to kill
SIGKILL failed to kill process
using fake rc=-1
program finished with exit code -1

remoteFailed: [Failure instance: Traceback from remote host -- Traceback (most recent call last):
Failure: exceptions.RuntimeError: SIGKILL failed to kill process
exceptions.RuntimeError: SIGKILL failed to kill process
]

[metajack]

Some initial Googling didn't turn up any results for pkill hanging. Hopefully this isn't going to happen often.

[metajack]

cc @edunham

[metajack]

We had to turn this off because it kept happening. See #146

[edunham]

The pkill step made it back in during the Buildbot refactor, and nobody has reported this issue continuing to happen, so I'm going to close this issue. If it is actually still a problem, please feel free to reopen with some recent examples.