Readonly access to session in a web service call?

We have a .net asmx web service that gets called from javascript (using ASP.Net AJAX), and requires access to Session.

[WebMethod(true)]
public string DoSomethingOnTheServer() { }

We're running into the problem of session being locked on a read/write request. Is there any way to mark a web service method as requiring read-only access to Session?

Thanks!