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Message-ID: <20191106165953.GF20975@paulmck-ThinkPad-P72>
Date: Wed, 6 Nov 2019 08:59:53 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Amol Grover <frextrite@...il.com>
Cc: Phong Tran <tranmanphong@...il.com>, linux-doc@...r.kernel.org,
corbet@....net, jiangshanlai@...il.com, josh@...htriplett.org,
rostedt@...dmis.org, linux-kernel@...r.kernel.org,
rcu@...r.kernel.org, mathieu.desnoyers@...icios.com,
joel@...lfernandes.org,
linux-kernel-mentees@...ts.linuxfoundation.org,
Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
Subject: Re: [PATCH] doc: Convert whatisRCU.txt to .rst
On Wed, Nov 06, 2019 at 08:48:24PM +0530, Amol Grover wrote:
> On Wed, Nov 06, 2019 at 08:09:50PM +0700, Phong Tran wrote:
> > This commit updates whatisRCU.txt to the new .rst format.
> > This change includes:
> >
> > - Formatting bullet lists
> > - Adding literal blocks
> > - Links from table of contents to corresponding sections
> > - Links to external documents
> > - Reformat quick quizzes
> >
> > Signed-off-by: Phong Tran <tranmanphong@...il.com>
> > Tested-by: Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
> > [ tranmanphong: Apply Amol Grover feedback. ]
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>
> Hey Phong,
> Tested it, everything is perfect!
> Reviewed-by: Amol Grover <frextrite@...il.com>
I replaced the commit in -rcu with this one, including Amol's
Reviewed-by. Thank you all!
Thanx, Paul
> Thanks
> Amol
>
> > ---
> > Documentation/RCU/index.rst | 1 +
> > .../RCU/{whatisRCU.txt => whatisRCU.rst} | 284 +++++++++++-------
> > 2 files changed, 178 insertions(+), 107 deletions(-)
> > rename Documentation/RCU/{whatisRCU.txt => whatisRCU.rst} (84%)
> >
> > diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
> > index 627128c230dc..b9b11481c727 100644
> > --- a/Documentation/RCU/index.rst
> > +++ b/Documentation/RCU/index.rst
> > @@ -8,6 +8,7 @@ RCU concepts
> > :maxdepth: 3
> >
> > arrayRCU
> > + whatisRCU
> > rcu
> > listRCU
> > NMI-RCU
> > diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.rst
> > similarity index 84%
> > rename from Documentation/RCU/whatisRCU.txt
> > rename to Documentation/RCU/whatisRCU.rst
> > index 58ba05c4d97f..2f6f6ebbc8b0 100644
> > --- a/Documentation/RCU/whatisRCU.txt
> > +++ b/Documentation/RCU/whatisRCU.rst
> > @@ -1,15 +1,18 @@
> > +.. _whatisrcu_doc:
> > +
> > What is RCU? -- "Read, Copy, Update"
> > +======================================
> >
> > Please note that the "What is RCU?" LWN series is an excellent place
> > to start learning about RCU:
> >
> > -1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
> > -2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
> > -3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
> > -4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
> > - 2010 Big API Table http://lwn.net/Articles/419086/
> > -5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/
> > - 2014 Big API Table http://lwn.net/Articles/609973/
> > +| 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
> > +| 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
> > +| 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
> > +| 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
> > +| 2010 Big API Table http://lwn.net/Articles/419086/
> > +| 5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/
> > +| 2014 Big API Table http://lwn.net/Articles/609973/
> >
> >
> > What is RCU?
> > @@ -24,14 +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
> > @@ -49,8 +59,10 @@ 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
> > +----------------
> >
> > The basic idea behind RCU is to split updates into "removal" and
> > "reclamation" phases. The removal phase removes references to data items
> > @@ -116,8 +128,10 @@ 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?
> > +---------------------------
> >
> > The core RCU API is quite small:
> >
> > @@ -136,7 +150,7 @@ later. See the kernel docbook documentation for more info, or look directly
> > at the function header comments.
> >
> > rcu_read_lock()
> > -
> > +^^^^^^^^^^^^^^^
> > void rcu_read_lock(void);
> >
> > Used by a reader to inform the reclaimer that the reader is
> > @@ -150,7 +164,7 @@ rcu_read_lock()
> > longer-term references to data structures.
> >
> > rcu_read_unlock()
> > -
> > +^^^^^^^^^^^^^^^^^
> > void rcu_read_unlock(void);
> >
> > Used by a reader to inform the reclaimer that the reader is
> > @@ -158,15 +172,15 @@ rcu_read_unlock()
> > read-side critical sections may be nested and/or overlapping.
> >
> > synchronize_rcu()
> > -
> > +^^^^^^^^^^^^^^^^^
> > void synchronize_rcu(void);
> >
> > Marks the end of updater code and the beginning of reclaimer
> > code. It does this by blocking until all pre-existing RCU
> > read-side critical sections on all CPUs have completed.
> > - Note that synchronize_rcu() will -not- necessarily wait for
> > + Note that synchronize_rcu() will **not** necessarily wait for
> > any subsequent RCU read-side critical sections to complete.
> > - For example, consider the following sequence of events:
> > + For example, consider the following sequence of events::
> >
> > CPU 0 CPU 1 CPU 2
> > ----------------- ------------------------- ---------------
> > @@ -182,7 +196,7 @@ synchronize_rcu()
> > any that begin after synchronize_rcu() is invoked.
> >
> > Of course, synchronize_rcu() does not necessarily return
> > - -immediately- after the last pre-existing RCU read-side critical
> > + **immediately** after the last pre-existing RCU read-side critical
> > section completes. For one thing, there might well be scheduling
> > delays. For another thing, many RCU implementations process
> > requests in batches in order to improve efficiencies, which can
> > @@ -211,10 +225,10 @@ synchronize_rcu()
> > checklist.txt for some approaches to limiting the update rate.
> >
> > rcu_assign_pointer()
> > -
> > +^^^^^^^^^^^^^^^^^^^^
> > void rcu_assign_pointer(p, typeof(p) v);
> >
> > - Yes, rcu_assign_pointer() -is- implemented as a macro, though it
> > + Yes, rcu_assign_pointer() **is** implemented as a macro, though it
> > would be cool to be able to declare a function in this manner.
> > (Compiler experts will no doubt disagree.)
> >
> > @@ -231,7 +245,7 @@ rcu_assign_pointer()
> > the _rcu list-manipulation primitives such as list_add_rcu().
> >
> > rcu_dereference()
> > -
> > +^^^^^^^^^^^^^^^^^
> > typeof(p) rcu_dereference(p);
> >
> > Like rcu_assign_pointer(), rcu_dereference() must be implemented
> > @@ -248,13 +262,13 @@ rcu_dereference()
> >
> > Common coding practice uses rcu_dereference() to copy an
> > RCU-protected pointer to a local variable, then dereferences
> > - this local variable, for example as follows:
> > + this local variable, for example as follows::
> >
> > p = rcu_dereference(head.next);
> > return p->data;
> >
> > However, in this case, one could just as easily combine these
> > - into one statement:
> > + into one statement::
> >
> > return rcu_dereference(head.next)->data;
> >
> > @@ -266,8 +280,8 @@ rcu_dereference()
> > unnecessary overhead on Alpha CPUs.
> >
> > Note that the value returned by rcu_dereference() is valid
> > - only within the enclosing RCU read-side critical section [1].
> > - For example, the following is -not- legal:
> > + only within the enclosing RCU read-side critical section [1]_.
> > + For example, the following is **not** legal::
> >
> > rcu_read_lock();
> > p = rcu_dereference(head.next);
> > @@ -290,9 +304,9 @@ rcu_dereference()
> > at any time, including immediately after the rcu_dereference().
> > And, again like rcu_assign_pointer(), rcu_dereference() is
> > typically used indirectly, via the _rcu list-manipulation
> > - primitives, such as list_for_each_entry_rcu() [2].
> > + primitives, such as list_for_each_entry_rcu() [2]_.
> >
> > - [1] The variant rcu_dereference_protected() can be used outside
> > +.. [1] The variant rcu_dereference_protected() can be used outside
> > of an RCU read-side critical section as long as the usage is
> > protected by locks acquired by the update-side code. This variant
> > avoids the lockdep warning that would happen when using (for
> > @@ -305,7 +319,7 @@ rcu_dereference()
> > a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst
> > and the API's code comments for more details and example usage.
> >
> > - [2] If the list_for_each_entry_rcu() instance might be used by
> > +.. [2] If the list_for_each_entry_rcu() instance might be used by
> > update-side code as well as by RCU readers, then an additional
> > lockdep expression can be added to its list of arguments.
> > For example, given an additional "lock_is_held(&mylock)" argument,
> > @@ -315,6 +329,7 @@ rcu_dereference()
> >
> > The following diagram shows how each API communicates among the
> > reader, updater, and reclaimer.
> > +::
> >
> >
> > rcu_assign_pointer()
> > @@ -375,12 +390,16 @@ 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 {
> > int a;
> > @@ -440,40 +459,43 @@ uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
> >
> > So, to sum up:
> >
> > -o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
> > +- Use rcu_read_lock() and rcu_read_unlock() to guard RCU
> > read-side critical sections.
> >
> > -o Within an RCU read-side critical section, use rcu_dereference()
> > +- Within an RCU read-side critical section, use rcu_dereference()
> > to dereference RCU-protected pointers.
> >
> > -o Use some solid scheme (such as locks or semaphores) to
> > +- Use some solid scheme (such as locks or semaphores) to
> > keep concurrent updates from interfering with each other.
> >
> > -o Use rcu_assign_pointer() to update an RCU-protected pointer.
> > +- Use rcu_assign_pointer() to update an RCU-protected pointer.
> > This primitive protects concurrent readers from the updater,
> > - -not- concurrent updates from each other! You therefore still
> > + **not** concurrent updates from each other! You therefore still
> > need to use locking (or something similar) to keep concurrent
> > rcu_assign_pointer() primitives from interfering with each other.
> >
> > -o Use synchronize_rcu() -after- removing a data element from an
> > - RCU-protected data structure, but -before- reclaiming/freeing
> > +- Use synchronize_rcu() **after** removing a data element from an
> > + RCU-protected data structure, but **before** reclaiming/freeing
> > the data element, in order to wait for the completion of all
> > RCU read-side critical sections that might be referencing that
> > 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?
> > +--------------------------------------------
> >
> > In the example above, foo_update_a() blocks until a grace period elapses.
> > This is quite simple, but in some cases one cannot afford to wait so
> > long -- there might be other high-priority work to be done.
> >
> > In such cases, one uses call_rcu() rather than synchronize_rcu().
> > -The call_rcu() API is as follows:
> > +The call_rcu() API is as follows::
> >
> > void call_rcu(struct rcu_head * head,
> > void (*func)(struct rcu_head *head));
> > @@ -481,7 +503,7 @@ The call_rcu() API is as follows:
> > This function invokes func(head) after a grace period has elapsed.
> > This invocation might happen from either softirq or process context,
> > so the function is not permitted to block. The foo struct needs to
> > -have an rcu_head structure added, perhaps as follows:
> > +have an rcu_head structure added, perhaps as follows::
> >
> > struct foo {
> > int a;
> > @@ -490,7 +512,7 @@ have an rcu_head structure added, perhaps as follows:
> > struct rcu_head rcu;
> > };
> >
> > -The foo_update_a() function might then be written as follows:
> > +The foo_update_a() function might then be written as follows::
> >
> > /*
> > * Create a new struct foo that is the same as the one currently
> > @@ -520,7 +542,7 @@ The foo_update_a() function might then be written as follows:
> > call_rcu(&old_fp->rcu, foo_reclaim);
> > }
> >
> > -The foo_reclaim() function might appear as follows:
> > +The foo_reclaim() function might appear as follows::
> >
> > void foo_reclaim(struct rcu_head *rp)
> > {
> > @@ -544,7 +566,7 @@ namely foo_reclaim().
> > The summary of advice is the same as for the previous section, except
> > that we are now using call_rcu() rather than synchronize_rcu():
> >
> > -o Use call_rcu() -after- removing a data element from an
> > +- Use call_rcu() **after** removing a data element from an
> > RCU-protected data structure in order to register a callback
> > function that will be invoked after the completion of all RCU
> > read-side critical sections that might be referencing that
> > @@ -552,14 +574,16 @@ o Use call_rcu() -after- removing a data element from an
> >
> > If the callback for call_rcu() is not doing anything more than calling
> > kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
> > -to avoid having to write your own callback:
> > +to avoid having to write your own callback::
> >
> > kfree_rcu(old_fp, rcu);
> >
> > Again, see checklist.txt for additional rules governing the use of RCU.
> >
> > +.. _5_whatisRCU:
> >
> > 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
> > +------------------------------------------------
> >
> > One of the nice things about RCU is that it has extremely simple "toy"
> > implementations that are a good first step towards understanding the
> > @@ -579,7 +603,7 @@ more details on the current implementation as of early 2004.
> >
> >
> > 5A. "TOY" IMPLEMENTATION #1: LOCKING
> > -
> > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > This section presents a "toy" RCU implementation that is based on
> > familiar locking primitives. Its overhead makes it a non-starter for
> > real-life use, as does its lack of scalability. It is also unsuitable
> > @@ -591,7 +615,7 @@ you allow nested rcu_read_lock() calls, you can deadlock.
> > However, it is probably the easiest implementation to relate to, so is
> > a good starting point.
> >
> > -It is extremely simple:
> > +It is extremely simple::
> >
> > static DEFINE_RWLOCK(rcu_gp_mutex);
> >
> > @@ -614,7 +638,7 @@ It is extremely simple:
> >
> > [You can ignore rcu_assign_pointer() and rcu_dereference() without missing
> > much. But here are simplified versions anyway. And whatever you do,
> > -don't forget about them when submitting patches making use of RCU!]
> > +don't forget about them when submitting patches making use of RCU!]::
> >
> > #define rcu_assign_pointer(p, v) \
> > ({ \
> > @@ -647,18 +671,23 @@ 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
> > +.. _quiz_1:
> > +
> > +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
> > -
> > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > This section presents a "toy" RCU implementation that is based on
> > "classic RCU". It is also short on performance (but only for updates) and
> > on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
> > kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
> > are the same as those shown in the preceding section, so they are omitted.
> > +::
> >
> > void rcu_read_lock(void) { }
> >
> > @@ -683,14 +712,14 @@ CPU in turn. The run_on() primitive can be implemented straightforwardly
> > in terms of the sched_setaffinity() primitive. Of course, a somewhat less
> > "toy" implementation would restore the affinity upon completion rather
> > than just leaving all tasks running on the last CPU, but when I said
> > -"toy", I meant -toy-!
> > +"toy", I meant **toy**!
> >
> > So how the heck is this supposed to work???
> >
> > Remember that it is illegal to block while in an RCU read-side critical
> > section. Therefore, if a given CPU executes a context switch, we know
> > that it must have completed all preceding RCU read-side critical sections.
> > -Once -all- CPUs have executed a context switch, then -all- preceding
> > +Once **all** CPUs have executed a context switch, then **all** preceding
> > RCU read-side critical sections will have completed.
> >
> > So, suppose that we remove a data item from its structure and then invoke
> > @@ -698,19 +727,32 @@ 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
> > - overhead is -negative-.
> > +.. _quiz_2:
> > +
> > +Quick Quiz #2:
> > + Give an example where Classic RCU's read-side
> > + overhead is **negative**.
> > +
> > +:ref:`Answers to Quick Quiz <8_whatisRCU>`
> >
> > -Quick Quiz #3: If it is illegal to block in an RCU read-side
> > +.. _quiz_3:
> > +
> > +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
> > +--------------------------------------
> >
> > Although RCU can be used in many different ways, a very common use of
> > RCU is analogous to reader-writer locking. The following unified
> > diff shows how closely related RCU and reader-writer locking can be.
> > +::
> >
> > @@ -5,5 +5,5 @@ struct el {
> > int data;
> > @@ -762,7 +804,7 @@ diff shows how closely related RCU and reader-writer locking can be.
> > return 0;
> > }
> >
> > -Or, for those who prefer a side-by-side listing:
> > +Or, for those who prefer a side-by-side listing::
> >
> > 1 struct el { 1 struct el {
> > 2 struct list_head list; 2 struct list_head list;
> > @@ -774,40 +816,44 @@ Or, for those who prefer a side-by-side listing:
> > 8 rwlock_t listmutex; 8 spinlock_t listmutex;
> > 9 struct el head; 9 struct el head;
> >
> > - 1 int search(long key, int *result) 1 int search(long key, int *result)
> > - 2 { 2 {
> > - 3 struct list_head *lp; 3 struct list_head *lp;
> > - 4 struct el *p; 4 struct el *p;
> > - 5 5
> > - 6 read_lock(&listmutex); 6 rcu_read_lock();
> > - 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
> > - 8 if (p->key == key) { 8 if (p->key == key) {
> > - 9 *result = p->data; 9 *result = p->data;
> > -10 read_unlock(&listmutex); 10 rcu_read_unlock();
> > -11 return 1; 11 return 1;
> > -12 } 12 }
> > -13 } 13 }
> > -14 read_unlock(&listmutex); 14 rcu_read_unlock();
> > -15 return 0; 15 return 0;
> > -16 } 16 }
> > -
> > - 1 int delete(long key) 1 int delete(long key)
> > - 2 { 2 {
> > - 3 struct el *p; 3 struct el *p;
> > - 4 4
> > - 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
> > - 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
> > - 7 if (p->key == key) { 7 if (p->key == key) {
> > - 8 list_del(&p->list); 8 list_del_rcu(&p->list);
> > - 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
> > - 10 synchronize_rcu();
> > -10 kfree(p); 11 kfree(p);
> > -11 return 1; 12 return 1;
> > -12 } 13 }
> > -13 } 14 }
> > -14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
> > -15 return 0; 16 return 0;
> > -16 } 17 }
> > +::
> > +
> > + 1 int search(long key, int *result) 1 int search(long key, int *result)
> > + 2 { 2 {
> > + 3 struct list_head *lp; 3 struct list_head *lp;
> > + 4 struct el *p; 4 struct el *p;
> > + 5 5
> > + 6 read_lock(&listmutex); 6 rcu_read_lock();
> > + 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
> > + 8 if (p->key == key) { 8 if (p->key == key) {
> > + 9 *result = p->data; 9 *result = p->data;
> > + 10 read_unlock(&listmutex); 10 rcu_read_unlock();
> > + 11 return 1; 11 return 1;
> > + 12 } 12 }
> > + 13 } 13 }
> > + 14 read_unlock(&listmutex); 14 rcu_read_unlock();
> > + 15 return 0; 15 return 0;
> > + 16 } 16 }
> > +
> > +::
> > +
> > + 1 int delete(long key) 1 int delete(long key)
> > + 2 { 2 {
> > + 3 struct el *p; 3 struct el *p;
> > + 4 4
> > + 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
> > + 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
> > + 7 if (p->key == key) { 7 if (p->key == key) {
> > + 8 list_del(&p->list); 8 list_del_rcu(&p->list);
> > + 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
> > + 10 synchronize_rcu();
> > + 10 kfree(p); 11 kfree(p);
> > + 11 return 1; 12 return 1;
> > + 12 } 13 }
> > + 13 } 14 }
> > + 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
> > + 15 return 0; 16 return 0;
> > + 16 } 17 }
> >
> > Either way, the differences are quite small. Read-side locking moves
> > to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
> > @@ -825,15 +871,17 @@ 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
> > +-------------------------
> >
> > The RCU APIs are documented in docbook-format header comments in the
> > Linux-kernel source code, but it helps to have a full list of the
> > APIs, since there does not appear to be a way to categorize them
> > in docbook. Here is the list, by category.
> >
> > -RCU list traversal:
> > +RCU list traversal::
> >
> > list_entry_rcu
> > list_first_entry_rcu
> > @@ -854,7 +902,7 @@ RCU list traversal:
> > hlist_bl_first_rcu
> > hlist_bl_for_each_entry_rcu
> >
> > -RCU pointer/list update:
> > +RCU pointer/list udate::
> >
> > rcu_assign_pointer
> > list_add_rcu
> > @@ -876,7 +924,9 @@ RCU pointer/list update:
> > hlist_bl_del_rcu
> > hlist_bl_set_first_rcu
> >
> > -RCU: Critical sections Grace period Barrier
> > +RCU::
> > +
> > + Critical sections Grace period Barrier
> >
> > rcu_read_lock synchronize_net rcu_barrier
> > rcu_read_unlock synchronize_rcu
> > @@ -885,7 +935,9 @@ RCU: Critical sections Grace period Barrier
> > rcu_dereference_check kfree_rcu
> > rcu_dereference_protected
> >
> > -bh: Critical sections Grace period Barrier
> > +bh::
> > +
> > + Critical sections Grace period Barrier
> >
> > rcu_read_lock_bh call_rcu rcu_barrier
> > rcu_read_unlock_bh synchronize_rcu
> > @@ -896,7 +948,9 @@ bh: Critical sections Grace period Barrier
> > rcu_dereference_bh_protected
> > rcu_read_lock_bh_held
> >
> > -sched: Critical sections Grace period Barrier
> > +sched::
> > +
> > + Critical sections Grace period Barrier
> >
> > rcu_read_lock_sched call_rcu rcu_barrier
> > rcu_read_unlock_sched synchronize_rcu
> > @@ -910,7 +964,9 @@ sched: Critical sections Grace period Barrier
> > rcu_read_lock_sched_held
> >
> >
> > -SRCU: Critical sections Grace period Barrier
> > +SRCU::
> > +
> > + Critical sections Grace period Barrier
> >
> > srcu_read_lock call_srcu srcu_barrier
> > srcu_read_unlock synchronize_srcu
> > @@ -918,13 +974,14 @@ SRCU: Critical sections Grace period Barrier
> > srcu_dereference_check
> > srcu_read_lock_held
> >
> > -SRCU: Initialization/cleanup
> > +SRCU: Initialization/cleanup::
> > +
> > DEFINE_SRCU
> > DEFINE_STATIC_SRCU
> > init_srcu_struct
> > cleanup_srcu_struct
> >
> > -All: lockdep-checked RCU-protected pointer access
> > +All: lockdep-checked RCU-protected pointer access::
> >
> > rcu_access_pointer
> > rcu_dereference_raw
> > @@ -974,15 +1031,19 @@ 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
> > @@ -1021,10 +1082,14 @@ 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
> > - overhead is -negative-.
> > +:ref:`Back to Quick Quiz #1 <quiz_1>`
> > +
> > +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
> > @@ -1046,11 +1111,15 @@ 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
> > +:ref:`Back to Quick Quiz #2 <quiz_2>`
> > +
> > +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
> > @@ -1069,6 +1138,7 @@ Answer: Just as PREEMPT_RT permits preemption of spinlock
> > Besides, how does the computer know what pizza parlor
> > the human being went to???
> >
> > +:ref:`Back to Quick Quiz #3 <quiz_3>`
> >
> > ACKNOWLEDGEMENTS
> >
> > --
> > 2.20.1
> >
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