doc: Update removal of RCU-bh/sched update machinery

The RCU-bh update API is now defined in terms of that of RCU-bh and
RCU-sched, so this commit updates the documentation accordingly.

In addition, although RCU-sched persists in !PREEMPT kernels, in
the PREEMPT case its update API is now defined in terms of that of
RCU-preempt, so this commit also updates the documentation accordingly.

While in the area, this commit removes the documentation for the
now-obsolete synchronize_rcu_mult() and clarifies the Tasks RCU
documentation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 35 insertions, 114 deletions

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 51f39f65002d..701b5c53607f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1306,8 +1306,6 @@ doing so would degrade real-time response.
 
 <p>
 This non-requirement appeared with preemptible RCU.
-If you need a grace period that waits on non-preemptible code regions, use
-<a href="#Sched Flavor">RCU-sched</a>.
 
 <h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
 
@@ -2165,14 +2163,9 @@ however, this is not a panacea because there would be severe restrictions
 on what operations those callbacks could invoke.
 
 <p>
-Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
-<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
-(<a href="#Bottom-Half Flavor">discussed below</a>),
-<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>,
+Perhaps surprisingly, <tt>synchronize_rcu()</tt> and
 <tt>synchronize_rcu_expedited()</tt>,
-<tt>synchronize_rcu_bh_expedited()</tt>, and
-<tt>synchronize_sched_expedited()</tt>
-will all operate normally
+will operate normally
 during very early boot, the reason being that there is only one CPU
 and preemption is disabled.
 This means that the call <tt>synchronize_rcu()</tt> (or friends)
@@ -2861,15 +2854,22 @@ The other four flavors are listed below, with requirements for each
 described in a separate section.
 
 <ol>
-<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
-<li>	<a href="#Sched Flavor">Sched Flavor</a>
+<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a>
+<li>	<a href="#Sched Flavor">Sched Flavor (Historical)</a>
 <li>	<a href="#Sleepable RCU">Sleepable RCU</a>
 <li>	<a href="#Tasks RCU">Tasks RCU</a>
-<li>	<a href="#Waiting for Multiple Grace Periods">
-	Waiting for Multiple Grace Periods</a>
 </ol>
 
-<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor (Historical)</a></h3>
+
+<p>
+The RCU-bh flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable softirq and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
 
 <p>
 The softirq-disable (AKA &ldquo;bottom-half&rdquo;,
@@ -2929,8 +2929,20 @@ includes
 <tt>call_rcu_bh()</tt>,
 <tt>rcu_barrier_bh()</tt>, and
 <tt>rcu_read_lock_bh_held()</tt>.
+However, the update-side APIs are now simple wrappers for other RCU
+flavors, namely RCU-sched in CONFIG_PREEMPT=n kernels and RCU-preempt
+otherwise.
 
-<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+<h3><a name="Sched Flavor">Sched Flavor (Historical)</a></h3>
+
+<p>
+The RCU-sched flavor of RCU has since been expressed in terms of
+the other RCU flavors as part of a consolidation of the three
+flavors into a single flavor.
+The read-side API remains, and continues to disable preemption and to
+be accounted for by lockdep.
+Much of the material in this section is therefore strictly historical
+in nature.
 
 <p>
 Before preemptible RCU, waiting for an RCU grace period had the
@@ -3150,94 +3162,14 @@ The tasks-RCU API is quite compact, consisting only of
 <tt>call_rcu_tasks()</tt>,
 <tt>synchronize_rcu_tasks()</tt>, and
 <tt>rcu_barrier_tasks()</tt>.
-
-<h3><a name="Waiting for Multiple Grace Periods">
-Waiting for Multiple Grace Periods</a></h3>
-
-<p>
-Perhaps you have an RCU protected data structure that is accessed from
-RCU read-side critical sections, from softirq handlers, and from
-hardware interrupt handlers.
-That is three flavors of RCU, the normal flavor, the bottom-half flavor,
-and the sched flavor.
-How to wait for a compound grace period?
-
-<p>
-The best approach is usually to &ldquo;just say no!&rdquo; and
-insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
-around each RCU read-side critical section, regardless of what
-environment it happens to be in.
-But suppose that some of the RCU read-side critical sections are
-on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
-is not a viable option, so that <tt>rcu_read_lock()</tt> and
-<tt>rcu_read_unlock()</tt> are not free.
-What then?
-
-<p>
-You <i>could</i> wait on all three grace periods in succession, as follows:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu();
- 2 synchronize_rcu_bh();
- 3 synchronize_sched();
-</pre>
-</blockquote>
-
-<p>
-This works, but triples the update-side latency penalty.
-In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
-may be used to wait on all three flavors of grace period concurrently:
-
-<blockquote>
-<pre>
- 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
-</pre>
-</blockquote>
-
-<p>
-But what if it is necessary to also wait on SRCU?
-This can be done as follows:
-
-<blockquote>
-<pre>
- 1 static void call_my_srcu(struct rcu_head *head,
- 2        void (*func)(struct rcu_head *head))
- 3 {
- 4   call_srcu(&amp;my_srcu, head, func);
- 5 }
- 6
- 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
-</pre>
-</blockquote>
-
-<p>
-If you needed to wait on multiple different flavors of SRCU
-(but why???), you would need to create a wrapper function resembling
-<tt>call_my_srcu()</tt> for each SRCU flavor.
-
-<table>
-<tr><th>&nbsp;</th></tr>
-<tr><th align="left">Quick Quiz:</th></tr>
-<tr><td>
-	But what if I need to wait for multiple RCU flavors, but I also need
-	the grace periods to be expedited?
-</td></tr>
-<tr><th align="left">Answer:</th></tr>
-<tr><td bgcolor="#ffffff"><font color="ffffff">
-	If you are using expedited grace periods, there should be less penalty
-	for waiting on them in succession.
-	But if that is nevertheless a problem, you can use workqueues
-	or multiple kthreads to wait on the various expedited grace
-	periods concurrently.
-</font></td></tr>
-<tr><td>&nbsp;</td></tr>
-</table>
-
-<p>
-Again, it is usually better to adjust the RCU read-side critical sections
-to use a single flavor of RCU, but when this is not feasible, you can use
-<tt>synchronize_rcu_mult()</tt>.
+In <tt>CONFIG_PREEMPT=n</tt> kernels, trampolines cannot be preempted,
+so these APIs map to
+<tt>call_rcu()</tt>,
+<tt>synchronize_rcu()</tt>, and
+<tt>rcu_barrier()</tt>, respectively.
+In <tt>CONFIG_PREEMPT=y</tt> kernels, trampolines can be preempted,
+and these three APIs are therefore implemented by separate functions
+that check for voluntary context switches.
 
 <h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
 
@@ -3249,12 +3181,6 @@ grace-period state machine so as to avoid the need for the additional
 latency.
 
 <p>
-Expedited grace periods scan the CPUs, so their latency and overhead
-increases with increasing numbers of CPUs.
-If this becomes a serious problem on large systems, it will be necessary
-to do some redesign to avoid this scalability problem.
-
-<p>
 RCU disables CPU hotplug in a few places, perhaps most notably in the
 <tt>rcu_barrier()</tt> operations.
 If there is a strong reason to use <tt>rcu_barrier()</tt> in CPU-hotplug
@@ -3299,11 +3225,6 @@ require extremely good demonstration of need and full exploration of
 alternatives.
 
 <p>
-There is an embarrassingly large number of flavors of RCU, and this
-number has been increasing over time.
-Perhaps it will be possible to combine some at some future date.
-
-<p>
 RCU's various kthreads are reasonably recent additions.
 It is quite likely that adjustments will be required to more gracefully
 handle extreme loads.