[PATCH] Doc: Improve format for whatisRCU.rst

Phong Tran
2019-11-02T11:55:36Z

Adding crossreference target for some headers, answer of quizzes

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
---
Documentation/RCU/whatisRCU.rst | 73 +++++++++++++++++++++++----------
1 file changed, 52 insertions(+), 21 deletions(-)

diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 70d0e4c21917..ae40c8bcc56c 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -1,4 +1,4 @@
-.. _rcu_doc:
+.. _whatisrcu_doc:

What is RCU? -- "Read, Copy, Update"
======================================
@@ -27,14 +27,21 @@ the experience has been that different people must take different paths
to arrive at an understanding of RCU. This document provides several
different paths, as follows:

-1. RCU OVERVIEW
-2. WHAT IS RCU'S CORE API?
-3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
-4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
-5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
-6. ANALOGY WITH READER-WRITER LOCKING
-7. FULL LIST OF RCU APIs
-8. ANSWERS TO QUICK QUIZZES
+:ref:`1. RCU OVERVIEW <1_whatisRCU>`
+
+:ref:`2. WHAT IS RCU'S CORE API? <2_whatisRCU>`
+
+:ref:`3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>`
+
+:ref:`4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>`
+
+:ref:`5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>`
+
+:ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
+
+:ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>`
+
+:ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>`

People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
@@ -52,6 +59,7 @@ everything, feel free to read the whole thing -- but if you are really
that type of person, you have perused the source code and will therefore
never need this document anyway. ;-)

+.. _1_whatisRCU:

1. RCU OVERVIEW
----------------
@@ -120,6 +128,7 @@ So how the heck can a reclaimer tell when a reader is done, given
that readers are not doing any sort of synchronization operations???
Read on to learn about how RCU's API makes this easy.

+.. _2_whatisRCU:

2. WHAT IS RCU'S CORE API?
---------------------------
@@ -381,13 +390,15 @@ c. RCU applied to scheduler and interrupt/NMI-handler tasks.
Again, most uses will be of (a). The (b) and (c) cases are important
for specialized uses, but are relatively uncommon.

+.. _3_whatisRCU:

3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
-----------------------------------------------

This section shows a simple use of the core RCU API to protect a
global pointer to a dynamically allocated structure. More-typical
-uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
+uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`,
+:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`.
::

struct foo {
@@ -470,9 +481,11 @@ o Use synchronize_rcu() -after- removing a data element from an
data item.

See checklist.txt for additional rules to follow when using RCU.
-And again, more-typical uses of RCU may be found in listRCU.txt,
-arrayRCU.txt, and NMI-RCU.txt.
+And again, more-typical uses of RCU may be found in :ref:`listRCU.rst
+<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst
+<NMI_rcu_doc>`.

+.. _4_whatisRCU:

4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
--------------------------------------------
@@ -567,6 +580,7 @@ to avoid having to write your own callback::

Again, see checklist.txt for additional rules governing the use of RCU.

+.. _5_whatisRCU:

5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
------------------------------------------------
@@ -657,10 +671,12 @@ that the only thing that can block rcu_read_lock() is a synchronize_rcu().
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
so there can be no deadlock cycle.

-Quick Quiz #1: Why is this argument naive? How could a deadlock
+Quick Quiz #1:
+ Why is this argument naive? How could a deadlock
occur when using this algorithm in a real-world Linux
kernel? How could this deadlock be avoided?

+:ref:`Answers to Quick Quiz <8_whatisRCU>`

5B. "TOY" EXAMPLE #2: CLASSIC RCU

@@ -709,13 +725,20 @@ synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
that there are no RCU read-side critical sections holding a reference
to that data item, so we can safely reclaim it.

-Quick Quiz #2: Give an example where Classic RCU's read-side
+Quick Quiz #2:
+ Give an example where Classic RCU's read-side
overhead is -negative-.

-Quick Quiz #3: If it is illegal to block in an RCU read-side
+:ref:`Answers to Quick Quiz <8_whatisRCU>`
+
+Quick Quiz #3:
+ If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
PREEMPT_RT, where normal spinlocks can block???

+:ref:`Answers to Quick Quiz <8_whatisRCU>`
+
+.. _6_whatisRCU:

6. ANALOGY WITH READER-WRITER LOCKING
--------------------------------------
@@ -842,6 +865,7 @@ delete() can now block. If this is a problem, there is a callback-based
mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
be used in place of synchronize_rcu().

+.. _7_whatisRCU:

7. FULL LIST OF RCU APIs
-------------------------
@@ -1001,16 +1025,19 @@ g. Otherwise, use RCU.
Of course, this all assumes that you have determined that RCU is in fact
the right tool for your job.

+.. _8_whatisRCU:

8. ANSWERS TO QUICK QUIZZES
----------------------------

-Quick Quiz #1: Why is this argument naive? How could a deadlock
+Quick Quiz #1:
+ Why is this argument naive? How could a deadlock
occur when using this algorithm in a real-world Linux
kernel? [Referring to the lock-based "toy" RCU
algorithm.]

-Answer: Consider the following sequence of events:
+Answer:
+ Consider the following sequence of events:

1. CPU 0 acquires some unrelated lock, call it
"problematic_lock", disabling irq via
@@ -1049,10 +1076,12 @@ Answer: Consider the following sequence of events:
approach where tasks in RCU read-side critical sections
cannot be blocked by tasks executing synchronize_rcu().

-Quick Quiz #2: Give an example where Classic RCU's read-side
+Quick Quiz #2:
+ Give an example where Classic RCU's read-side
overhead is -negative-.

-Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
+Answer:
+ Imagine a single-CPU system with a non-CONFIG_PREEMPT
kernel where a routing table is used by process-context
code, but can be updated by irq-context code (for example,
by an "ICMP REDIRECT" packet). The usual way of handling
@@ -1074,11 +1103,13 @@ Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
even the theoretical possibility of negative overhead for
a synchronization primitive is a bit unexpected. ;-)

-Quick Quiz #3: If it is illegal to block in an RCU read-side
+Quick Quiz #3:
+ If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
PREEMPT_RT, where normal spinlocks can block???

-Answer: Just as PREEMPT_RT permits preemption of spinlock
+Answer:
+ Just as PREEMPT_RT permits preemption of spinlock
critical sections, it permits preemption of RCU
read-side critical sections. It also permits
spinlocks blocking while in RCU read-side critical
--
2.20.1