Thread-safe Rails controller actions - setting instance variables?

I have to write a threaded Rails app because I am running it atop of Neo4j.rb, which embeds a Neo4j graph database inside the Rails process, and thus I have to serve multiple requests from the same process. Yeah, it'd be cool if connecting to a Neo4j database worked like SQL databases, but it doesn't, so I'll quit complaining and just use it.

I'm quite worried about the implications of writing concurrent code (as I should be), and just need some advice on how to handle common a common scenario - a controller sets an instance variable or a variable in the session hash, then some stuff happens. Consider the following crude code to demonstrate what I mean:

# THIS IS NOT REAL PRODUCTION CODE
# I don't do this in real life, it is just to help me ask my question, I
# know about one-way hashing, etc.!

class SessionsController
  def create
    user = User.find_by_email_and_password(params[:email], params[:password])
    raise 'auth error' unless user
    session[:current_user_id] = user.id
    redirect_to :controller => 'current_user', :action => 'show'
  end
end

class CurrentUserController
  def show
    @current_user = User.find(session[:current_user_id])
    render :action => :show # .html.erb file that uses @current_user
  end
end

The question: Are there any race conditions in this code?

In SessionsController, are the session hash and the params hash thread-local? Say the same browser session makes multiple requests to /sessions#create (to borrow Rails route syntax) with different credentials, the user that is logged in should be the request that hit the line session[:current_user_id] = user.id last? Or should I wrap a mutex lock around the controller action?

In the CurrentUserController, if the show action is hit simultaneously by two requests with different sessions, will the same @current_user variable be set by both? I.e. will the first request, as it is processing the .html.erb file, find that it's @current_user instance variable has suddenly been changed by the second thread?

Thanks