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Message-ID: <20191102161825.GR20975@paulmck-ThinkPad-P72>
Date: Sat, 2 Nov 2019 09:18:25 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Phong Tran <tranmanphong@...il.com>
Cc: josh@...htriplett.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
joel@...lfernandes.org, corbet@....net,
madhuparnabhowmik04@...il.com, rcu@...r.kernel.org,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kernel-mentees@...ts.linuxfoundation.org,
skhan@...uxfoundation.org
Subject: Re: [PATCH] Doc: Improve format for whatisRCU.rst
On Sat, Nov 02, 2019 at 06:55:17PM +0700, Phong Tran wrote:
> Adding crossreference target for some headers, answer of quizzes
>
> Signed-off-by: Phong Tran <tranmanphong@...il.com>
I have queued this, thank you!
Once Madhuparna reviews/tests, I will merge it into your earlier version.
Thanx, Paul
> ---
> 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
>
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