Restore docs "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()"

This restores docs back in ReST format.

Signed-off-by: Joel Fernandes (Google) <[e-mail]>
[ paulmck: Added Joel's SoB per Stephen Rothwell feedback. ]
[ paulmck: Joel approved via private email. ]
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/Design/Requirements/Requirements.rst

···
 #. `Hotplug CPU`_
 #. `Scheduler and RCU`_
 #. `Tracing and RCU`_
+#. `Accesses to User Memory and RCU`_
 #. `Energy Efficiency`_
 #. `Scheduling-Clock Interrupts and RCU`_
 #. `Memory Efficiency`_
···
 where RCU readers execute in environments in which tracing cannot be
 used. The tracing folks both located the requirement and provided the
 needed fix, so this surprise requirement was relatively painless.
+
+Accesses to User Memory and RCU
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The kernel needs to access user-space memory, for example, to access data
+referenced by system-call parameters.  The ``get_user()`` macro does this job.
+
+However, user-space memory might well be paged out, which means that
+``get_user()`` might well page-fault and thus block while waiting for the
+resulting I/O to complete.  It would be a very bad thing for the compiler to
+reorder a ``get_user()`` invocation into an RCU read-side critical section.
+
+For example, suppose that the source code looked like this:
+
+  ::
+
+       1 rcu_read_lock();
+       2 p = rcu_dereference(gp);
+       3 v = p->value;
+       4 rcu_read_unlock();
+       5 get_user(user_v, user_p);
+       6 do_something_with(v, user_v);
+
+The compiler must not be permitted to transform this source code into
+the following:
+
+  ::
+
+       1 rcu_read_lock();
+       2 p = rcu_dereference(gp);
+       3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
+       4 v = p->value;
+       5 rcu_read_unlock();
+       6 do_something_with(v, user_v);
+
+If the compiler did make this transformation in a ``CONFIG_PREEMPT=n`` kernel
+build, and if ``get_user()`` did page fault, the result would be a quiescent
+state in the middle of an RCU read-side critical section.  This misplaced
+quiescent state could result in line 4 being a use-after-free access,
+which could be bad for your kernel's actuarial statistics.  Similar examples
+can be constructed with the call to ``get_user()`` preceding the
+``rcu_read_lock()``.
+
+Unfortunately, ``get_user()`` doesn't have any particular ordering properties,
+and in some architectures the underlying ``asm`` isn't even marked
+``volatile``.  And even if it was marked ``volatile``, the above access to
+``p->value`` is not volatile, so the compiler would not have any reason to keep
+those two accesses in order.
+
+Therefore, the Linux-kernel definitions of ``rcu_read_lock()`` and
+``rcu_read_unlock()`` must act as compiler barriers, at least for outermost
+instances of ``rcu_read_lock()`` and ``rcu_read_unlock()`` within a nested set
+of RCU read-side critical sections.
 
 Energy Efficiency
 ~~~~~~~~~~~~~~~~~