[PATCH] Document Linux's memory barriers [try #4]

[Eric W. Biederman]

A small nit.  You are not documenting the most subtle memory barrier:
smp_read_barrier_depends();  Which is a deep requirement of the RCU
code.

As I understand it.  On some architectures (alpha) without at least
this a load from a pointer can load from the an old pointer value.

At one point it was suggested this be called: 
read_memory_barrier_data_dependent().

Simply calling: rcu_dereference is what all users should call but
the semantics should be documented at least so that people porting
Linux can have a chance of getting it right.

Eric