[PATCH tip/core/rcu 16/20] doc: Remove obsolete (non-)requirement about disabling preemption

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>

The Requirements.html document says "Disabling Preemption Does
Not Block Grace Periods".  However this is no longer true with
the RCU consolidation.  This commit therefore removes the obsolete
(non-)requirement entirely.

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 .../RCU/Design/Requirements/Requirements.html | 50 -------------------
 1 file changed, 50 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 7efc1c1da7af..4fae55056c1d 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -900,8 +900,6 @@ Except where otherwise noted, these non-guarantees were premeditated.
 	Grace Periods Don't Partition Read-Side Critical Sections</a>
 <li>	<a href="#Read-Side Critical Sections Don't Partition Grace Periods">
 	Read-Side Critical Sections Don't Partition Grace Periods</a>
-<li>	<a href="#Disabling Preemption Does Not Block Grace Periods">
-	Disabling Preemption Does Not Block Grace Periods</a>
 </ol>
 
 <h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
@@ -1259,54 +1257,6 @@ of RCU grace periods.
 <tr><td>&nbsp;</td></tr>
 </table>
 
-<h3><a name="Disabling Preemption Does Not Block Grace Periods">
-Disabling Preemption Does Not Block Grace Periods</a></h3>
-
-<p>
-There was a time when disabling preemption on any given CPU would block
-subsequent grace periods.
-However, this was an accident of implementation and is not a requirement.
-And in the current Linux-kernel implementation, disabling preemption
-on a given CPU in fact does not block grace periods, as Oleg Nesterov
-<a href="https://lkml.kernel.org/g/20150614193825.GA19582@xxxxxxxxxx";>demonstrated</a>.
-
-<p>
-If you need a preempt-disable region to block grace periods, you need to add
-<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
-as follows:
-
-<blockquote>
-<pre>
- 1 preempt_disable();
- 2 rcu_read_lock();
- 3 do_something();
- 4 rcu_read_unlock();
- 5 preempt_enable();
- 6
- 7 /* Spinlocks implicitly disable preemption. */
- 8 spin_lock(&amp;mylock);
- 9 rcu_read_lock();
-10 do_something();
-11 rcu_read_unlock();
-12 spin_unlock(&amp;mylock);
-</pre>
-</blockquote>
-
-<p>
-In theory, you could enter the RCU read-side critical section first,
-but it is more efficient to keep the entire RCU read-side critical
-section contained in the preempt-disable region as shown above.
-Of course, RCU read-side critical sections that extend outside of
-preempt-disable regions will work correctly, but such critical sections
-can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
-more work.
-And no, this is <i>not</i> an invitation to enclose all of your RCU
-read-side critical sections within preempt-disable regions, because
-doing so would degrade real-time response.
-
-<p>
-This non-requirement appeared with preemptible RCU.
-
 <h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
 
 <p>
-- 
2.17.1