How to lock the node.js function?

I am using embedded array in mongoDB, Document looks same as below

{
    "_id" : ObjectId("516f89b393307a0f73533863"),
    "desc" : "LrOBVgWHHRyAxgYdKIwwgATcGTNBlz",
    "comments" :[
                 { 
                     "comment" : "superb",
                     "likeOrUnlike" : [
                          {"name":"kk",
                           "date":"2013/04/11",
                            "like":true
                           }
                      ]
                 }
      ]
   "date" : ISODate("2013-04-18T05:50:43.112Z"),
   "title" : "OaTHxDDMoINSaZCHCSNpJLNQyrqXGZ"

}

I want to update the date and like field in the likeOrUnlike array inside the comments, Automic update is not possible in this scenario, so i thought of doing it manually by code(retrieving whole document and update),

In this case concurrency issue will occur

scenario : If two persons likes the same comment at same time.

So the solution is to lock the particular update function

 var updateComments = function(id,data){
     db.posts.findOne({"_id" :id},function(err,res){
        // stuff   
        //mongo update
     });
 };

I can't post entire code.

My Question is

How to lock the function in node.js?

locking node.js function is good practice?