[Click] write handler synchronization

[Roman Chertov]

> Yes.  Simply mark the handler as NONEXCLUSIVE using the 
> Handler::NONEXCLUSIVE flag.  Then you'll need to handle synchronization, 
> but the handler won't block.
> 

Excellent :)  Just one last question.  Are there limitations on what 
synchronization primitives I can use?  Especially, spinlock vs. a mutex. 
     The data copy from the clickfs into the kernel space takes place 
before the handler is called, so there should be no restriction of using 
a spinlock, right?

Roman