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

This reverts docs from commit d6b9cd7dc8e041ee83cb1362fce59a3cdb1f2709.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Added Joel's SoB per Stephen Rothwell feedback. ]
[ paulmck: Joel approved via private email. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/Design/Requirements/Requirements.html

@@ -2129,8 +2129,6 @@ Some of the relevant points of interest are as follows:
 <li>	<a href="#Hotplug CPU">Hotplug CPU</a>.
 <li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
 <li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
-<li>	<a href="#Accesses to User Memory and RCU">
-Accesses to User Memory and RCU</a>.
 <li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
 <li>	<a href="#Scheduling-Clock Interrupts and RCU">
 	Scheduling-Clock Interrupts and RCU</a>.
@@ -2523,75 +2521,6 @@ cannot be used.
 The tracing folks both located the requirement and provided the
 needed fix, so this surprise requirement was relatively painless.
 
-<h3><a name="Accesses to User Memory and RCU">
-Accesses to User Memory and RCU</a></h3>
-
-<p>
-The kernel needs to access user-space memory, for example, to access
-data referenced by system-call parameters.
-The <tt>get_user()</tt> macro does this job.
-
-<p>
-However, user-space memory might well be paged out, which means
-that <tt>get_user()</tt> 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 <tt>get_user()</tt> invocation into an RCU read-side critical
-section.
-For example, suppose that the source code looked like this:
-
-<blockquote>
-<pre>
- 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 v = p-&gt;value;
- 4 rcu_read_unlock();
- 5 get_user(user_v, user_p);
- 6 do_something_with(v, user_v);
-</pre>
-</blockquote>
-
-<p>
-The compiler must not be permitted to transform this source code into
-the following:
-
-<blockquote>
-<pre>
- 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
- 4 v = p-&gt;value;
- 5 rcu_read_unlock();
- 6 do_something_with(v, user_v);
-</pre>
-</blockquote>
-
-<p>
-If the compiler did make this transformation in a
-<tt>CONFIG_PREEMPT=n</tt> kernel build, and if <tt>get_user()</tt> 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&nbsp;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 <tt>get_user()</tt>
-preceding the <tt>rcu_read_lock()</tt>.
-
-<p>
-Unfortunately, <tt>get_user()</tt> doesn't have any particular
-ordering properties, and in some architectures the underlying <tt>asm</tt>
-isn't even marked <tt>volatile</tt>.
-And even if it was marked <tt>volatile</tt>, the above access to
-<tt>p-&gt;value</tt> is not volatile, so the compiler would not have any
-reason to keep those two accesses in order.
-
-<p>
-Therefore, the Linux-kernel definitions of <tt>rcu_read_lock()</tt>
-and <tt>rcu_read_unlock()</tt> must act as compiler barriers,
-at least for outermost instances of <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> within a nested set of RCU read-side critical
-sections.
-
 <h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
 
 <p>