Existing lock /var/run/yum.pid: another copy is running as pid

Existing lock /var/run/yum.pid: another copy is running as pid [number].
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: yum
    Memory :  23 M RSS ( 37 MB VSZ)
    Started: Sun Jul 10 12:38:40 2016 - 01:08 ago
    State  : Sleeping, pid: [number]

How to fix it?

root# kill -9 [number]

Example:

root# kill -9 5115

[Errno 14] PYCURL ERROR 22

Sometimes, you can get this error:

[Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
...
[Errno 14] PYCURL ERROR 9 - "Server denied you to change to the given directory"
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: sl-addons. Please verify its path and try again

But, don't  worry, server was turned off temporalily.

SLC CERN 6.x Repositories in the Scientific Linux 6.x

1. Download sl-release file:

i386: http://linuxsoft.cern.ch/cern/slc6X/i386/yum/updates/repoview/sl-release.html
  Or,
x86_64: http://linuxsoft.cern.ch/cern/slc6X/x86_64/yum/updates/repoview/sl-release.html

2. Open package with file roller, and extract files.

3. Open folder, and go to the this package's location: [package] /etc/yum.repos.d/

And you'll see these, and other repositories files:

slc6-extras.repo
slc6-os.repo
slc6-testing.repo
slc6-updates.repo

4. Copy these files to your system, open terminal as root in the [package] /etc/yum.repos.d/ location, and copy files:

root# cp slc6-extras.repo slc6-os.repo slc6-testing.repo slc6-updates.repo /etc/yum.repos.d/

5. After these steps, we can try to use slc6x repositories.

root#: yum check-update