Mongo error : Could not lock auth data update lock. on root user i have a mongodb on 2.6.9.
my connected user is root:

mongo --username myadmin --password thepassword --port  27018 --host 127.0.0.1 admin
MongoDB shell version: 2.6.9
connecting to: 127.0.0.1:27018/admin
mongos&gt; db.getUsers()
[
{
    "_id" : "admin.myadmin",
    "user" : "myadmin",
    "db" : "admin",
    "roles" : [
        {
            "role" : "root",
            "db" : "admin"
        },
        {
            "role" : "dbAdminAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "readAnyDatabase",
            "db" : "admin"
        },
        {
            "role" : "clusterAdmin",
            "db" : "admin"
        }
    ]
},


So i am root, but when i want update a user:

mongos&gt; db = db.getSiblingDB('myuser')
myuser
mongos&gt; db.updateUser(  'myuser' ,{ pwd: 'hg76vZa091', roles: ['dbOwner','dbAdmin','readWrite'] } )
2015-10-06T09:00:00.100+0000 Error: Updating user failed: Could not lock auth data update lock. at src/mongo/shell/db.js:1155


So i try to grant userAdmin to myadmin, and it's not working.

mongos&gt; use admin
mongos&gt; db.grantRolesToUser(     "myadmin",     [       { role: "userAdminAnyDatabase", db: "admin" }     ] )
2015-10-06T08:46:40.987+0000 Error: Could not lock auth data update lock. at src/mongo/shell/db.js:1287


Root user cannot grand user ? cannot update user ? How i can do it ?
Maybe i miss some query in my script. I try 

use admin


or 
    use root 

before grant or update user, but it's not working. Any idea?
You are actually using root and admin database by your query 
'use root' or 'use admin'

use root will not change your user it will just change your database.

So to add user or role .you need to login by root.As you have mentioned in very first line actually you are login as "myadmin " on database "admin"