[PATCH] Documentation: RCU: rcubarrier: Convert to reST

[PATCH] Documentation: RCU: rcubarrier: Convert to reST

[Amol Grover]

Convert rcubarrier.txt to rcubarrier.rst and
add it to index.rst

Format file according to reST
- Add headings and sub-headings
- Add code segments
- Add cross-references to quizes and answers

Signed-off-by: Amol Grover <frextrite@gmail.com>
---
 Documentation/RCU/index.rst                   |   1 +
 .../RCU/{rcubarrier.txt => rcubarrier.rst}    | 220 ++++++++++--------
 2 files changed, 125 insertions(+), 96 deletions(-)
 rename Documentation/RCU/{rcubarrier.txt => rcubarrier.rst} (73%)

diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index c81d0e4fd999..81a0a1e5f767 100644
--- a/Documentation/RCU/index.rst
+++ b/Documentation/RCU/index.rst
@@ -8,6 +8,7 @@ RCU concepts
    :maxdepth: 3
 
    arrayRCU
+   rcubarrier
    rcu_dereference
    whatisRCU
    rcu
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.rst
similarity index 73%
rename from Documentation/RCU/rcubarrier.txt
rename to Documentation/RCU/rcubarrier.rst
index a2782df69732..1aa9ed1d1b5b 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.rst
@@ -1,4 +1,7 @@
+.. _rcu_barrier:
+
 RCU and Unloadable Modules
+==========================
 
 [Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
 
@@ -21,7 +24,7 @@ given that readers might well leave absolutely no trace of their
 presence? There is a synchronize_rcu() primitive that blocks until all
 pre-existing readers have completed. An updater wishing to delete an
 element p from a linked list might do the following, while holding an
-appropriate lock, of course:
+appropriate lock, of course::
 
 	list_del_rcu(p);
 	synchronize_rcu();
@@ -32,13 +35,13 @@ primitive must be used instead. This primitive takes a pointer to an
 rcu_head struct placed within the RCU-protected data structure and
 another pointer to a function that may be invoked later to free that
 structure. Code to delete an element p from the linked list from IRQ
-context might then be as follows:
+context might then be as follows::
 
 	list_del_rcu(p);
 	call_rcu(&p->rcu, p_callback);
 
 Since call_rcu() never blocks, this code can safely be used from within
-IRQ context. The function p_callback() might be defined as follows:
+IRQ context. The function p_callback() might be defined as follows::
 
 	static void p_callback(struct rcu_head *rp)
 	{
@@ -49,6 +52,7 @@ IRQ context. The function p_callback() might be defined as follows:
 
 
 Unloading Modules That Use call_rcu()
+-------------------------------------
 
 But what if p_callback is defined in an unloadable module?
 
@@ -69,10 +73,11 @@ in realtime kernels in order to avoid excessive scheduling latencies.
 
 
 rcu_barrier()
+-------------
 
 We instead need the rcu_barrier() primitive.  Rather than waiting for
 a grace period to elapse, rcu_barrier() waits for all outstanding RCU
-callbacks to complete.  Please note that rcu_barrier() does -not- imply
+callbacks to complete.  Please note that rcu_barrier() does **not** imply
 synchronize_rcu(), in particular, if there are no RCU callbacks queued
 anywhere, rcu_barrier() is within its rights to return immediately,
 without waiting for a grace period to elapse.
@@ -89,78 +94,78 @@ module uses multiple flavors of call_rcu(), then it must also use multiple
 flavors of rcu_barrier() when unloading that module.  For example, if
 it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
 srcu_struct_2(), then the following three lines of code will be required
-when unloading:
+when unloading::
 
  1 rcu_barrier();
  2 srcu_barrier(&srcu_struct_1);
  3 srcu_barrier(&srcu_struct_2);
 
 The rcutorture module makes use of rcu_barrier() in its exit function
-as follows:
+as follows::
 
- 1 static void
- 2 rcu_torture_cleanup(void)
- 3 {
- 4   int i;
+ 1  static void
+ 2  rcu_torture_cleanup(void)
+ 3  {
+ 4    int i;
  5
- 6   fullstop = 1;
- 7   if (shuffler_task != NULL) {
+ 6    fullstop = 1;
+ 7    if (shuffler_task != NULL) {
  8     VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
  9     kthread_stop(shuffler_task);
-10   }
-11   shuffler_task = NULL;
-12
-13   if (writer_task != NULL) {
-14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
-15     kthread_stop(writer_task);
-16   }
-17   writer_task = NULL;
-18
-19   if (reader_tasks != NULL) {
-20     for (i = 0; i < nrealreaders; i++) {
-21       if (reader_tasks[i] != NULL) {
-22         VERBOSE_PRINTK_STRING(
-23           "Stopping rcu_torture_reader task");
-24         kthread_stop(reader_tasks[i]);
-25       }
-26       reader_tasks[i] = NULL;
-27     }
-28     kfree(reader_tasks);
-29     reader_tasks = NULL;
-30   }
-31   rcu_torture_current = NULL;
-32
-33   if (fakewriter_tasks != NULL) {
-34     for (i = 0; i < nfakewriters; i++) {
-35       if (fakewriter_tasks[i] != NULL) {
-36         VERBOSE_PRINTK_STRING(
-37           "Stopping rcu_torture_fakewriter task");
-38         kthread_stop(fakewriter_tasks[i]);
-39       }
-40       fakewriter_tasks[i] = NULL;
-41     }
-42     kfree(fakewriter_tasks);
-43     fakewriter_tasks = NULL;
-44   }
-45
-46   if (stats_task != NULL) {
-47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
-48     kthread_stop(stats_task);
-49   }
-50   stats_task = NULL;
-51
-52   /* Wait for all RCU callbacks to fire. */
-53   rcu_barrier();
-54
-55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
-56
-57   if (cur_ops->cleanup != NULL)
-58     cur_ops->cleanup();
-59   if (atomic_read(&n_rcu_torture_error))
-60     rcu_torture_print_module_parms("End of test: FAILURE");
-61   else
-62     rcu_torture_print_module_parms("End of test: SUCCESS");
-63 }
+ 10   }
+ 11   shuffler_task = NULL;
+ 12
+ 13   if (writer_task != NULL) {
+ 14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
+ 15     kthread_stop(writer_task);
+ 16   }
+ 17   writer_task = NULL;
+ 18
+ 19   if (reader_tasks != NULL) {
+ 20     for (i = 0; i < nrealreaders; i++) {
+ 21       if (reader_tasks[i] != NULL) {
+ 22         VERBOSE_PRINTK_STRING(
+ 23           "Stopping rcu_torture_reader task");
+ 24         kthread_stop(reader_tasks[i]);
+ 25       }
+ 26       reader_tasks[i] = NULL;
+ 27     }
+ 28     kfree(reader_tasks);
+ 29     reader_tasks = NULL;
+ 30   }
+ 31   rcu_torture_current = NULL;
+ 32
+ 33   if (fakewriter_tasks != NULL) {
+ 34     for (i = 0; i < nfakewriters; i++) {
+ 35       if (fakewriter_tasks[i] != NULL) {
+ 36         VERBOSE_PRINTK_STRING(
+ 37           "Stopping rcu_torture_fakewriter task");
+ 38         kthread_stop(fakewriter_tasks[i]);
+ 39       }
+ 40       fakewriter_tasks[i] = NULL;
+ 41     }
+ 42     kfree(fakewriter_tasks);
+ 43     fakewriter_tasks = NULL;
+ 44   }
+ 45
+ 46   if (stats_task != NULL) {
+ 47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
+ 48     kthread_stop(stats_task);
+ 49   }
+ 50   stats_task = NULL;
+ 51
+ 52   /* Wait for all RCU callbacks to fire. */
+ 53   rcu_barrier();
+ 54
+ 55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
+ 56
+ 57   if (cur_ops->cleanup != NULL)
+ 58     cur_ops->cleanup();
+ 59   if (atomic_read(&n_rcu_torture_error))
+ 60     rcu_torture_print_module_parms("End of test: FAILURE");
+ 61   else
+ 62     rcu_torture_print_module_parms("End of test: SUCCESS");
+ 63 }
 
 Line 6 sets a global variable that prevents any RCU callbacks from
 re-posting themselves. This will not be necessary in most cases, since
@@ -176,9 +181,14 @@ for any pre-existing callbacks to complete.
 Then lines 55-62 print status and do operation-specific cleanup, and
 then return, permitting the module-unload operation to be completed.
 
-Quick Quiz #1: Is there any other situation where rcu_barrier() might
+.. _rcubarrier_quiz_1:
+
+Quick Quiz #1:
+	Is there any other situation where rcu_barrier() might
 	be required?
 
+:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
+
 Your module might have additional complications. For example, if your
 module invokes call_rcu() from timers, you will need to first cancel all
 the timers, and only then invoke rcu_barrier() to wait for any remaining
@@ -188,11 +198,12 @@ Of course, if you module uses call_rcu(), you will need to invoke
 rcu_barrier() before unloading.  Similarly, if your module uses
 call_srcu(), you will need to invoke srcu_barrier() before unloading,
 and on the same srcu_struct structure.  If your module uses call_rcu()
--and- call_srcu(), then you will need to invoke rcu_barrier() -and-
+-and- call_srcu(), then you will need to invoke rcu_barrier() **and**
 srcu_barrier().
 
 
 Implementing rcu_barrier()
+--------------------------
 
 Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
 that RCU callbacks are never reordered once queued on one of the per-CPU
@@ -200,19 +211,19 @@ queues. His implementation queues an RCU callback on each of the per-CPU
 callback queues, and then waits until they have all started executing, at
 which point, all earlier RCU callbacks are guaranteed to have completed.
 
-The original code for rcu_barrier() was as follows:
+The original code for rcu_barrier() was as follows::
 
- 1 void rcu_barrier(void)
- 2 {
- 3   BUG_ON(in_interrupt());
- 4   /* Take cpucontrol mutex to protect against CPU hotplug */
- 5   mutex_lock(&rcu_barrier_mutex);
- 6   init_completion(&rcu_barrier_completion);
- 7   atomic_set(&rcu_barrier_cpu_count, 0);
- 8   on_each_cpu(rcu_barrier_func, NULL, 0, 1);
- 9   wait_for_completion(&rcu_barrier_completion);
-10   mutex_unlock(&rcu_barrier_mutex);
-11 }
+ 1  void rcu_barrier(void)
+ 2  {
+ 3    BUG_ON(in_interrupt());
+ 4    /* Take cpucontrol mutex to protect against CPU hotplug */
+ 5    mutex_lock(&rcu_barrier_mutex);
+ 6    init_completion(&rcu_barrier_completion);
+ 7    atomic_set(&rcu_barrier_cpu_count, 0);
+ 8    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
+ 9    wait_for_completion(&rcu_barrier_completion);
+ 10   mutex_unlock(&rcu_barrier_mutex);
+ 11 }
 
 Line 3 verifies that the caller is in process context, and lines 5 and 10
 use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
@@ -226,18 +237,18 @@ This code was rewritten in 2008 and several times thereafter, but this
 still gives the general idea.
 
 The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
-to post an RCU callback, as follows:
+to post an RCU callback, as follows::
 
- 1 static void rcu_barrier_func(void *notused)
- 2 {
- 3 int cpu = smp_processor_id();
- 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
- 5 struct rcu_head *head;
+ 1  static void rcu_barrier_func(void *notused)
+ 2  {
+ 3    int cpu = smp_processor_id();
+ 4    struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+ 5    struct rcu_head *head;
  6
- 7 head = &rdp->barrier;
- 8 atomic_inc(&rcu_barrier_cpu_count);
- 9 call_rcu(head, rcu_barrier_callback);
-10 }
+ 7    head = &rdp->barrier;
+ 8    atomic_inc(&rcu_barrier_cpu_count);
+ 9    call_rcu(head, rcu_barrier_callback);
+ 10 }
 
 Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
 which contains the struct rcu_head that needed for the later call to
@@ -248,20 +259,25 @@ the current CPU's queue.
 
 The rcu_barrier_callback() function simply atomically decrements the
 rcu_barrier_cpu_count variable and finalizes the completion when it
-reaches zero, as follows:
+reaches zero, as follows::
 
  1 static void rcu_barrier_callback(struct rcu_head *notused)
  2 {
- 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
- 4 complete(&rcu_barrier_completion);
+ 3   if (atomic_dec_and_test(&rcu_barrier_cpu_count))
+ 4     complete(&rcu_barrier_completion);
  5 }
 
-Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
+.. _rcubarrier_quiz_2:
+
+Quick Quiz #2:
+	What happens if CPU 0's rcu_barrier_func() executes
 	immediately (thus incrementing rcu_barrier_cpu_count to the
 	value one), but the other CPU's rcu_barrier_func() invocations
 	are delayed for a full grace period? Couldn't this result in
 	rcu_barrier() returning prematurely?
 
+:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
+
 The current rcu_barrier() implementation is more complex, due to the need
 to avoid disturbing idle CPUs (especially on battery-powered systems)
 and the need to minimally disturb non-idle CPUs in real-time systems.
@@ -269,6 +285,7 @@ However, the code above illustrates the concepts.
 
 
 rcu_barrier() Summary
+---------------------
 
 The rcu_barrier() primitive has seen relatively little use, since most
 code using RCU is in the core kernel rather than in modules. However, if
@@ -277,8 +294,12 @@ so that your module may be safely unloaded.
 
 
 Answers to Quick Quizzes
+------------------------
+
+.. _answer_rcubarrier_quiz_1:
 
-Quick Quiz #1: Is there any other situation where rcu_barrier() might
+Quick Quiz #1:
+	Is there any other situation where rcu_barrier() might
 	be required?
 
 Answer: Interestingly enough, rcu_barrier() was not originally
@@ -292,7 +313,12 @@ Answer: Interestingly enough, rcu_barrier() was not originally
 	implementing rcutorture, and found that rcu_barrier() solves
 	this problem as well.
 
-Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
+:ref:`Back to Quick Quiz #1 <rcubarrier_quiz_1>`
+
+.. _answer_rcubarrier_quiz_2:
+
+Quick Quiz #2:
+	What happens if CPU 0's rcu_barrier_func() executes
 	immediately (thus incrementing rcu_barrier_cpu_count to the
 	value one), but the other CPU's rcu_barrier_func() invocations
 	are delayed for a full grace period? Couldn't this result in
@@ -323,3 +349,5 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last
 	is to add an rcu_read_lock() before line 8 of rcu_barrier()
 	and an rcu_read_unlock() after line 8 of this same function. If
 	you can think of a better change, please let me know!
+
+:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
-- 
2.20.1

Re: [PATCH] Documentation: RCU: rcubarrier: Convert to reST

[Paul E. McKenney]

On Wed, Nov 06, 2019 at 10:26:17PM +0530, Amol Grover wrote:
> Convert rcubarrier.txt to rcubarrier.rst and
> add it to index.rst
> 
> Format file according to reST
> - Add headings and sub-headings
> - Add code segments
> - Add cross-references to quizes and answers
> 
> Signed-off-by: Amol Grover <frextrite@gmail.com>

Thank you, Amol!

Phong and Madhuparna, could you please look at this one?  I will hold
off queuing it for a couple of days to allow you to look it over.

							Thanx, Paul

> ---
>  Documentation/RCU/index.rst                   |   1 +
>  .../RCU/{rcubarrier.txt => rcubarrier.rst}    | 220 ++++++++++--------
>  2 files changed, 125 insertions(+), 96 deletions(-)
>  rename Documentation/RCU/{rcubarrier.txt => rcubarrier.rst} (73%)
> 
> diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
> index c81d0e4fd999..81a0a1e5f767 100644
> --- a/Documentation/RCU/index.rst
> +++ b/Documentation/RCU/index.rst
> @@ -8,6 +8,7 @@ RCU concepts
>     :maxdepth: 3
>  
>     arrayRCU
> +   rcubarrier
>     rcu_dereference
>     whatisRCU
>     rcu
> diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.rst
> similarity index 73%
> rename from Documentation/RCU/rcubarrier.txt
> rename to Documentation/RCU/rcubarrier.rst
> index a2782df69732..1aa9ed1d1b5b 100644
> --- a/Documentation/RCU/rcubarrier.txt
> +++ b/Documentation/RCU/rcubarrier.rst
> @@ -1,4 +1,7 @@
> +.. _rcu_barrier:
> +
>  RCU and Unloadable Modules
> +==========================
>  
>  [Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
>  
> @@ -21,7 +24,7 @@ given that readers might well leave absolutely no trace of their
>  presence? There is a synchronize_rcu() primitive that blocks until all
>  pre-existing readers have completed. An updater wishing to delete an
>  element p from a linked list might do the following, while holding an
> -appropriate lock, of course:
> +appropriate lock, of course::
>  
>  	list_del_rcu(p);
>  	synchronize_rcu();
> @@ -32,13 +35,13 @@ primitive must be used instead. This primitive takes a pointer to an
>  rcu_head struct placed within the RCU-protected data structure and
>  another pointer to a function that may be invoked later to free that
>  structure. Code to delete an element p from the linked list from IRQ
> -context might then be as follows:
> +context might then be as follows::
>  
>  	list_del_rcu(p);
>  	call_rcu(&p->rcu, p_callback);
>  
>  Since call_rcu() never blocks, this code can safely be used from within
> -IRQ context. The function p_callback() might be defined as follows:
> +IRQ context. The function p_callback() might be defined as follows::
>  
>  	static void p_callback(struct rcu_head *rp)
>  	{
> @@ -49,6 +52,7 @@ IRQ context. The function p_callback() might be defined as follows:
>  
>  
>  Unloading Modules That Use call_rcu()
> +-------------------------------------
>  
>  But what if p_callback is defined in an unloadable module?
>  
> @@ -69,10 +73,11 @@ in realtime kernels in order to avoid excessive scheduling latencies.
>  
>  
>  rcu_barrier()
> +-------------
>  
>  We instead need the rcu_barrier() primitive.  Rather than waiting for
>  a grace period to elapse, rcu_barrier() waits for all outstanding RCU
> -callbacks to complete.  Please note that rcu_barrier() does -not- imply
> +callbacks to complete.  Please note that rcu_barrier() does **not** imply
>  synchronize_rcu(), in particular, if there are no RCU callbacks queued
>  anywhere, rcu_barrier() is within its rights to return immediately,
>  without waiting for a grace period to elapse.
> @@ -89,78 +94,78 @@ module uses multiple flavors of call_rcu(), then it must also use multiple
>  flavors of rcu_barrier() when unloading that module.  For example, if
>  it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
>  srcu_struct_2(), then the following three lines of code will be required
> -when unloading:
> +when unloading::
>  
>   1 rcu_barrier();
>   2 srcu_barrier(&srcu_struct_1);
>   3 srcu_barrier(&srcu_struct_2);
>  
>  The rcutorture module makes use of rcu_barrier() in its exit function
> -as follows:
> +as follows::
>  
> - 1 static void
> - 2 rcu_torture_cleanup(void)
> - 3 {
> - 4   int i;
> + 1  static void
> + 2  rcu_torture_cleanup(void)
> + 3  {
> + 4    int i;
>   5
> - 6   fullstop = 1;
> - 7   if (shuffler_task != NULL) {
> + 6    fullstop = 1;
> + 7    if (shuffler_task != NULL) {
>   8     VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
>   9     kthread_stop(shuffler_task);
> -10   }
> -11   shuffler_task = NULL;
> -12
> -13   if (writer_task != NULL) {
> -14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> -15     kthread_stop(writer_task);
> -16   }
> -17   writer_task = NULL;
> -18
> -19   if (reader_tasks != NULL) {
> -20     for (i = 0; i < nrealreaders; i++) {
> -21       if (reader_tasks[i] != NULL) {
> -22         VERBOSE_PRINTK_STRING(
> -23           "Stopping rcu_torture_reader task");
> -24         kthread_stop(reader_tasks[i]);
> -25       }
> -26       reader_tasks[i] = NULL;
> -27     }
> -28     kfree(reader_tasks);
> -29     reader_tasks = NULL;
> -30   }
> -31   rcu_torture_current = NULL;
> -32
> -33   if (fakewriter_tasks != NULL) {
> -34     for (i = 0; i < nfakewriters; i++) {
> -35       if (fakewriter_tasks[i] != NULL) {
> -36         VERBOSE_PRINTK_STRING(
> -37           "Stopping rcu_torture_fakewriter task");
> -38         kthread_stop(fakewriter_tasks[i]);
> -39       }
> -40       fakewriter_tasks[i] = NULL;
> -41     }
> -42     kfree(fakewriter_tasks);
> -43     fakewriter_tasks = NULL;
> -44   }
> -45
> -46   if (stats_task != NULL) {
> -47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> -48     kthread_stop(stats_task);
> -49   }
> -50   stats_task = NULL;
> -51
> -52   /* Wait for all RCU callbacks to fire. */
> -53   rcu_barrier();
> -54
> -55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> -56
> -57   if (cur_ops->cleanup != NULL)
> -58     cur_ops->cleanup();
> -59   if (atomic_read(&n_rcu_torture_error))
> -60     rcu_torture_print_module_parms("End of test: FAILURE");
> -61   else
> -62     rcu_torture_print_module_parms("End of test: SUCCESS");
> -63 }
> + 10   }
> + 11   shuffler_task = NULL;
> + 12
> + 13   if (writer_task != NULL) {
> + 14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> + 15     kthread_stop(writer_task);
> + 16   }
> + 17   writer_task = NULL;
> + 18
> + 19   if (reader_tasks != NULL) {
> + 20     for (i = 0; i < nrealreaders; i++) {
> + 21       if (reader_tasks[i] != NULL) {
> + 22         VERBOSE_PRINTK_STRING(
> + 23           "Stopping rcu_torture_reader task");
> + 24         kthread_stop(reader_tasks[i]);
> + 25       }
> + 26       reader_tasks[i] = NULL;
> + 27     }
> + 28     kfree(reader_tasks);
> + 29     reader_tasks = NULL;
> + 30   }
> + 31   rcu_torture_current = NULL;
> + 32
> + 33   if (fakewriter_tasks != NULL) {
> + 34     for (i = 0; i < nfakewriters; i++) {
> + 35       if (fakewriter_tasks[i] != NULL) {
> + 36         VERBOSE_PRINTK_STRING(
> + 37           "Stopping rcu_torture_fakewriter task");
> + 38         kthread_stop(fakewriter_tasks[i]);
> + 39       }
> + 40       fakewriter_tasks[i] = NULL;
> + 41     }
> + 42     kfree(fakewriter_tasks);
> + 43     fakewriter_tasks = NULL;
> + 44   }
> + 45
> + 46   if (stats_task != NULL) {
> + 47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> + 48     kthread_stop(stats_task);
> + 49   }
> + 50   stats_task = NULL;
> + 51
> + 52   /* Wait for all RCU callbacks to fire. */
> + 53   rcu_barrier();
> + 54
> + 55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> + 56
> + 57   if (cur_ops->cleanup != NULL)
> + 58     cur_ops->cleanup();
> + 59   if (atomic_read(&n_rcu_torture_error))
> + 60     rcu_torture_print_module_parms("End of test: FAILURE");
> + 61   else
> + 62     rcu_torture_print_module_parms("End of test: SUCCESS");
> + 63 }
>  
>  Line 6 sets a global variable that prevents any RCU callbacks from
>  re-posting themselves. This will not be necessary in most cases, since
> @@ -176,9 +181,14 @@ for any pre-existing callbacks to complete.
>  Then lines 55-62 print status and do operation-specific cleanup, and
>  then return, permitting the module-unload operation to be completed.
>  
> -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> +.. _rcubarrier_quiz_1:
> +
> +Quick Quiz #1:
> +	Is there any other situation where rcu_barrier() might
>  	be required?
>  
> +:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
> +
>  Your module might have additional complications. For example, if your
>  module invokes call_rcu() from timers, you will need to first cancel all
>  the timers, and only then invoke rcu_barrier() to wait for any remaining
> @@ -188,11 +198,12 @@ Of course, if you module uses call_rcu(), you will need to invoke
>  rcu_barrier() before unloading.  Similarly, if your module uses
>  call_srcu(), you will need to invoke srcu_barrier() before unloading,
>  and on the same srcu_struct structure.  If your module uses call_rcu()
> --and- call_srcu(), then you will need to invoke rcu_barrier() -and-
> +-and- call_srcu(), then you will need to invoke rcu_barrier() **and**
>  srcu_barrier().
>  
>  
>  Implementing rcu_barrier()
> +--------------------------
>  
>  Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
>  that RCU callbacks are never reordered once queued on one of the per-CPU
> @@ -200,19 +211,19 @@ queues. His implementation queues an RCU callback on each of the per-CPU
>  callback queues, and then waits until they have all started executing, at
>  which point, all earlier RCU callbacks are guaranteed to have completed.
>  
> -The original code for rcu_barrier() was as follows:
> +The original code for rcu_barrier() was as follows::
>  
> - 1 void rcu_barrier(void)
> - 2 {
> - 3   BUG_ON(in_interrupt());
> - 4   /* Take cpucontrol mutex to protect against CPU hotplug */
> - 5   mutex_lock(&rcu_barrier_mutex);
> - 6   init_completion(&rcu_barrier_completion);
> - 7   atomic_set(&rcu_barrier_cpu_count, 0);
> - 8   on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> - 9   wait_for_completion(&rcu_barrier_completion);
> -10   mutex_unlock(&rcu_barrier_mutex);
> -11 }
> + 1  void rcu_barrier(void)
> + 2  {
> + 3    BUG_ON(in_interrupt());
> + 4    /* Take cpucontrol mutex to protect against CPU hotplug */
> + 5    mutex_lock(&rcu_barrier_mutex);
> + 6    init_completion(&rcu_barrier_completion);
> + 7    atomic_set(&rcu_barrier_cpu_count, 0);
> + 8    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> + 9    wait_for_completion(&rcu_barrier_completion);
> + 10   mutex_unlock(&rcu_barrier_mutex);
> + 11 }
>  
>  Line 3 verifies that the caller is in process context, and lines 5 and 10
>  use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
> @@ -226,18 +237,18 @@ This code was rewritten in 2008 and several times thereafter, but this
>  still gives the general idea.
>  
>  The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
> -to post an RCU callback, as follows:
> +to post an RCU callback, as follows::
>  
> - 1 static void rcu_barrier_func(void *notused)
> - 2 {
> - 3 int cpu = smp_processor_id();
> - 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> - 5 struct rcu_head *head;
> + 1  static void rcu_barrier_func(void *notused)
> + 2  {
> + 3    int cpu = smp_processor_id();
> + 4    struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> + 5    struct rcu_head *head;
>   6
> - 7 head = &rdp->barrier;
> - 8 atomic_inc(&rcu_barrier_cpu_count);
> - 9 call_rcu(head, rcu_barrier_callback);
> -10 }
> + 7    head = &rdp->barrier;
> + 8    atomic_inc(&rcu_barrier_cpu_count);
> + 9    call_rcu(head, rcu_barrier_callback);
> + 10 }
>  
>  Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
>  which contains the struct rcu_head that needed for the later call to
> @@ -248,20 +259,25 @@ the current CPU's queue.
>  
>  The rcu_barrier_callback() function simply atomically decrements the
>  rcu_barrier_cpu_count variable and finalizes the completion when it
> -reaches zero, as follows:
> +reaches zero, as follows::
>  
>   1 static void rcu_barrier_callback(struct rcu_head *notused)
>   2 {
> - 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> - 4 complete(&rcu_barrier_completion);
> + 3   if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> + 4     complete(&rcu_barrier_completion);
>   5 }
>  
> -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> +.. _rcubarrier_quiz_2:
> +
> +Quick Quiz #2:
> +	What happens if CPU 0's rcu_barrier_func() executes
>  	immediately (thus incrementing rcu_barrier_cpu_count to the
>  	value one), but the other CPU's rcu_barrier_func() invocations
>  	are delayed for a full grace period? Couldn't this result in
>  	rcu_barrier() returning prematurely?
>  
> +:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
> +
>  The current rcu_barrier() implementation is more complex, due to the need
>  to avoid disturbing idle CPUs (especially on battery-powered systems)
>  and the need to minimally disturb non-idle CPUs in real-time systems.
> @@ -269,6 +285,7 @@ However, the code above illustrates the concepts.
>  
>  
>  rcu_barrier() Summary
> +---------------------
>  
>  The rcu_barrier() primitive has seen relatively little use, since most
>  code using RCU is in the core kernel rather than in modules. However, if
> @@ -277,8 +294,12 @@ so that your module may be safely unloaded.
>  
>  
>  Answers to Quick Quizzes
> +------------------------
> +
> +.. _answer_rcubarrier_quiz_1:
>  
> -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> +Quick Quiz #1:
> +	Is there any other situation where rcu_barrier() might
>  	be required?
>  
>  Answer: Interestingly enough, rcu_barrier() was not originally
> @@ -292,7 +313,12 @@ Answer: Interestingly enough, rcu_barrier() was not originally
>  	implementing rcutorture, and found that rcu_barrier() solves
>  	this problem as well.
>  
> -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> +:ref:`Back to Quick Quiz #1 <rcubarrier_quiz_1>`
> +
> +.. _answer_rcubarrier_quiz_2:
> +
> +Quick Quiz #2:
> +	What happens if CPU 0's rcu_barrier_func() executes
>  	immediately (thus incrementing rcu_barrier_cpu_count to the
>  	value one), but the other CPU's rcu_barrier_func() invocations
>  	are delayed for a full grace period? Couldn't this result in
> @@ -323,3 +349,5 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last
>  	is to add an rcu_read_lock() before line 8 of rcu_barrier()
>  	and an rcu_read_unlock() after line 8 of this same function. If
>  	you can think of a better change, please let me know!
> +
> +:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
> -- 
> 2.20.1
> 

Re: [Linux-kernel-mentees] [PATCH] Documentation: RCU: rcubarrier: Convert to reST

[Phong Tran]

On 11/6/19 11:56 PM, Amol Grover wrote:
> Convert rcubarrier.txt to rcubarrier.rst and
> add it to index.rst
> 
> Format file according to reST
> - Add headings and sub-headings
> - Add code segments
> - Add cross-references to quizes and answers
> 
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>   Documentation/RCU/index.rst                   |   1 +
>   .../RCU/{rcubarrier.txt => rcubarrier.rst}    | 220 ++++++++++--------
>   2 files changed, 125 insertions(+), 96 deletions(-)
>   rename Documentation/RCU/{rcubarrier.txt => rcubarrier.rst} (73%)
> 
> diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
> index c81d0e4fd999..81a0a1e5f767 100644
> --- a/Documentation/RCU/index.rst
> +++ b/Documentation/RCU/index.rst
> @@ -8,6 +8,7 @@ RCU concepts
>      :maxdepth: 3
>   
>      arrayRCU
> +   rcubarrier
>      rcu_dereference
>      whatisRCU
>      rcu
> diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.rst
> similarity index 73%
> rename from Documentation/RCU/rcubarrier.txt
> rename to Documentation/RCU/rcubarrier.rst
> index a2782df69732..1aa9ed1d1b5b 100644
> --- a/Documentation/RCU/rcubarrier.txt
> +++ b/Documentation/RCU/rcubarrier.rst
> @@ -1,4 +1,7 @@
> +.. _rcu_barrier:
> +
>   RCU and Unloadable Modules
> +==========================
>   
>   [Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
>   
> @@ -21,7 +24,7 @@ given that readers might well leave absolutely no trace of their
>   presence? There is a synchronize_rcu() primitive that blocks until all
>   pre-existing readers have completed. An updater wishing to delete an
>   element p from a linked list might do the following, while holding an
> -appropriate lock, of course:
> +appropriate lock, of course::
>   
>   	list_del_rcu(p);
>   	synchronize_rcu();
> @@ -32,13 +35,13 @@ primitive must be used instead. This primitive takes a pointer to an
>   rcu_head struct placed within the RCU-protected data structure and
>   another pointer to a function that may be invoked later to free that
>   structure. Code to delete an element p from the linked list from IRQ
> -context might then be as follows:
> +context might then be as follows::
>   
>   	list_del_rcu(p);
>   	call_rcu(&p->rcu, p_callback);
>   
>   Since call_rcu() never blocks, this code can safely be used from within
> -IRQ context. The function p_callback() might be defined as follows:
> +IRQ context. The function p_callback() might be defined as follows::
>   
>   	static void p_callback(struct rcu_head *rp)
>   	{
> @@ -49,6 +52,7 @@ IRQ context. The function p_callback() might be defined as follows:
>   
>   
>   Unloading Modules That Use call_rcu()
> +-------------------------------------
>   
>   But what if p_callback is defined in an unloadable module?
>   
> @@ -69,10 +73,11 @@ in realtime kernels in order to avoid excessive scheduling latencies.
>   
>   
>   rcu_barrier()
> +-------------
>   
>   We instead need the rcu_barrier() primitive.  Rather than waiting for
>   a grace period to elapse, rcu_barrier() waits for all outstanding RCU
> -callbacks to complete.  Please note that rcu_barrier() does -not- imply
> +callbacks to complete.  Please note that rcu_barrier() does **not** imply
>   synchronize_rcu(), in particular, if there are no RCU callbacks queued
>   anywhere, rcu_barrier() is within its rights to return immediately,
>   without waiting for a grace period to elapse.
> @@ -89,78 +94,78 @@ module uses multiple flavors of call_rcu(), then it must also use multiple
>   flavors of rcu_barrier() when unloading that module.  For example, if
>   it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
>   srcu_struct_2(), then the following three lines of code will be required

Hello Amol,

srcu_struct_2() should be srcu_struct_2

> -when unloading:
> +when unloading::
>   
>    1 rcu_barrier();
>    2 srcu_barrier(&srcu_struct_1);
>    3 srcu_barrier(&srcu_struct_2);
>   
>   The rcutorture module makes use of rcu_barrier() in its exit function
> -as follows:
> +as follows::
>   
> - 1 static void
> - 2 rcu_torture_cleanup(void)
> - 3 {
> - 4   int i;
> + 1  static void
> + 2  rcu_torture_cleanup(void)
> + 3  {
> + 4    int i;
>    5
> - 6   fullstop = 1;
> - 7   if (shuffler_task != NULL) {
> + 6    fullstop = 1;
> + 7    if (shuffler_task != NULL) {
>    8     VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
>    9     kthread_stop(shuffler_task);
> -10   }
> -11   shuffler_task = NULL;
> -12
> -13   if (writer_task != NULL) {
> -14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> -15     kthread_stop(writer_task);
> -16   }
> -17   writer_task = NULL;
> -18
> -19   if (reader_tasks != NULL) {
> -20     for (i = 0; i < nrealreaders; i++) {
> -21       if (reader_tasks[i] != NULL) {
> -22         VERBOSE_PRINTK_STRING(
> -23           "Stopping rcu_torture_reader task");
> -24         kthread_stop(reader_tasks[i]);
> -25       }
> -26       reader_tasks[i] = NULL;
> -27     }
> -28     kfree(reader_tasks);
> -29     reader_tasks = NULL;
> -30   }
> -31   rcu_torture_current = NULL;
> -32
> -33   if (fakewriter_tasks != NULL) {
> -34     for (i = 0; i < nfakewriters; i++) {
> -35       if (fakewriter_tasks[i] != NULL) {
> -36         VERBOSE_PRINTK_STRING(
> -37           "Stopping rcu_torture_fakewriter task");
> -38         kthread_stop(fakewriter_tasks[i]);
> -39       }
> -40       fakewriter_tasks[i] = NULL;
> -41     }
> -42     kfree(fakewriter_tasks);
> -43     fakewriter_tasks = NULL;
> -44   }
> -45
> -46   if (stats_task != NULL) {
> -47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> -48     kthread_stop(stats_task);
> -49   }
> -50   stats_task = NULL;
> -51
> -52   /* Wait for all RCU callbacks to fire. */
> -53   rcu_barrier();
> -54
> -55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> -56
> -57   if (cur_ops->cleanup != NULL)
> -58     cur_ops->cleanup();
> -59   if (atomic_read(&n_rcu_torture_error))
> -60     rcu_torture_print_module_parms("End of test: FAILURE");
> -61   else
> -62     rcu_torture_print_module_parms("End of test: SUCCESS");
> -63 }
> + 10   }
> + 11   shuffler_task = NULL;
> + 12
> + 13   if (writer_task != NULL) {
> + 14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> + 15     kthread_stop(writer_task);
> + 16   }
> + 17   writer_task = NULL;
> + 18
> + 19   if (reader_tasks != NULL) {
> + 20     for (i = 0; i < nrealreaders; i++) {
> + 21       if (reader_tasks[i] != NULL) {
> + 22         VERBOSE_PRINTK_STRING(
> + 23           "Stopping rcu_torture_reader task");
> + 24         kthread_stop(reader_tasks[i]);
> + 25       }
> + 26       reader_tasks[i] = NULL;
> + 27     }
> + 28     kfree(reader_tasks);
> + 29     reader_tasks = NULL;
> + 30   }
> + 31   rcu_torture_current = NULL;
> + 32
> + 33   if (fakewriter_tasks != NULL) {
> + 34     for (i = 0; i < nfakewriters; i++) {
> + 35       if (fakewriter_tasks[i] != NULL) {
> + 36         VERBOSE_PRINTK_STRING(
> + 37           "Stopping rcu_torture_fakewriter task");
> + 38         kthread_stop(fakewriter_tasks[i]);
> + 39       }
> + 40       fakewriter_tasks[i] = NULL;
> + 41     }
> + 42     kfree(fakewriter_tasks);
> + 43     fakewriter_tasks = NULL;
> + 44   }
> + 45
> + 46   if (stats_task != NULL) {
> + 47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> + 48     kthread_stop(stats_task);
> + 49   }
> + 50   stats_task = NULL;
> + 51
> + 52   /* Wait for all RCU callbacks to fire. */
> + 53   rcu_barrier();
> + 54
> + 55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> + 56
> + 57   if (cur_ops->cleanup != NULL)
> + 58     cur_ops->cleanup();
> + 59   if (atomic_read(&n_rcu_torture_error))
> + 60     rcu_torture_print_module_parms("End of test: FAILURE");
> + 61   else
> + 62     rcu_torture_print_module_parms("End of test: SUCCESS");
> + 63 }
>   
>   Line 6 sets a global variable that prevents any RCU callbacks from
>   re-posting themselves. This will not be necessary in most cases, since
> @@ -176,9 +181,14 @@ for any pre-existing callbacks to complete.
>   Then lines 55-62 print status and do operation-specific cleanup, and
>   then return, permitting the module-unload operation to be completed.
>   
> -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> +.. _rcubarrier_quiz_1:
> +
> +Quick Quiz #1:
> +	Is there any other situation where rcu_barrier() might
>   	be required?
>   
> +:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
> +
>   Your module might have additional complications. For example, if your
>   module invokes call_rcu() from timers, you will need to first cancel all
>   the timers, and only then invoke rcu_barrier() to wait for any remaining
> @@ -188,11 +198,12 @@ Of course, if you module uses call_rcu(), you will need to invoke
>   rcu_barrier() before unloading.  Similarly, if your module uses
>   call_srcu(), you will need to invoke srcu_barrier() before unloading,
>   and on the same srcu_struct structure.  If your module uses call_rcu()
> --and- call_srcu(), then you will need to invoke rcu_barrier() -and-
> +-and- call_srcu(), then you will need to invoke rcu_barrier() **and**

-and- here should be bold.

The rest looks good.

Regards,
Phong.

>   srcu_barrier().
>   
>   
>   Implementing rcu_barrier()
> +--------------------------
>   
>   Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
>   that RCU callbacks are never reordered once queued on one of the per-CPU
> @@ -200,19 +211,19 @@ queues. His implementation queues an RCU callback on each of the per-CPU
>   callback queues, and then waits until they have all started executing, at
>   which point, all earlier RCU callbacks are guaranteed to have completed.
>   
> -The original code for rcu_barrier() was as follows:
> +The original code for rcu_barrier() was as follows::
>   
> - 1 void rcu_barrier(void)
> - 2 {
> - 3   BUG_ON(in_interrupt());
> - 4   /* Take cpucontrol mutex to protect against CPU hotplug */
> - 5   mutex_lock(&rcu_barrier_mutex);
> - 6   init_completion(&rcu_barrier_completion);
> - 7   atomic_set(&rcu_barrier_cpu_count, 0);
> - 8   on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> - 9   wait_for_completion(&rcu_barrier_completion);
> -10   mutex_unlock(&rcu_barrier_mutex);
> -11 }
> + 1  void rcu_barrier(void)
> + 2  {
> + 3    BUG_ON(in_interrupt());
> + 4    /* Take cpucontrol mutex to protect against CPU hotplug */
> + 5    mutex_lock(&rcu_barrier_mutex);
> + 6    init_completion(&rcu_barrier_completion);
> + 7    atomic_set(&rcu_barrier_cpu_count, 0);
> + 8    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> + 9    wait_for_completion(&rcu_barrier_completion);
> + 10   mutex_unlock(&rcu_barrier_mutex);
> + 11 }
>   
>   Line 3 verifies that the caller is in process context, and lines 5 and 10
>   use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
> @@ -226,18 +237,18 @@ This code was rewritten in 2008 and several times thereafter, but this
>   still gives the general idea.
>   
>   The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
> -to post an RCU callback, as follows:
> +to post an RCU callback, as follows::
>   
> - 1 static void rcu_barrier_func(void *notused)
> - 2 {
> - 3 int cpu = smp_processor_id();
> - 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> - 5 struct rcu_head *head;
> + 1  static void rcu_barrier_func(void *notused)
> + 2  {
> + 3    int cpu = smp_processor_id();
> + 4    struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> + 5    struct rcu_head *head;
>    6
> - 7 head = &rdp->barrier;
> - 8 atomic_inc(&rcu_barrier_cpu_count);
> - 9 call_rcu(head, rcu_barrier_callback);
> -10 }
> + 7    head = &rdp->barrier;
> + 8    atomic_inc(&rcu_barrier_cpu_count);
> + 9    call_rcu(head, rcu_barrier_callback);
> + 10 }
>   
>   Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
>   which contains the struct rcu_head that needed for the later call to
> @@ -248,20 +259,25 @@ the current CPU's queue.
>   
>   The rcu_barrier_callback() function simply atomically decrements the
>   rcu_barrier_cpu_count variable and finalizes the completion when it
> -reaches zero, as follows:
> +reaches zero, as follows::
>   
>    1 static void rcu_barrier_callback(struct rcu_head *notused)
>    2 {
> - 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> - 4 complete(&rcu_barrier_completion);
> + 3   if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> + 4     complete(&rcu_barrier_completion);
>    5 }
>   
> -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> +.. _rcubarrier_quiz_2:
> +
> +Quick Quiz #2:
> +	What happens if CPU 0's rcu_barrier_func() executes
>   	immediately (thus incrementing rcu_barrier_cpu_count to the
>   	value one), but the other CPU's rcu_barrier_func() invocations
>   	are delayed for a full grace period? Couldn't this result in
>   	rcu_barrier() returning prematurely?
>   
> +:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
> +
>   The current rcu_barrier() implementation is more complex, due to the need
>   to avoid disturbing idle CPUs (especially on battery-powered systems)
>   and the need to minimally disturb non-idle CPUs in real-time systems.
> @@ -269,6 +285,7 @@ However, the code above illustrates the concepts.
>   
>   
>   rcu_barrier() Summary
> +---------------------
>   
>   The rcu_barrier() primitive has seen relatively little use, since most
>   code using RCU is in the core kernel rather than in modules. However, if
> @@ -277,8 +294,12 @@ so that your module may be safely unloaded.
>   
>   
>   Answers to Quick Quizzes
> +------------------------
> +
> +.. _answer_rcubarrier_quiz_1:
>   
> -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> +Quick Quiz #1:
> +	Is there any other situation where rcu_barrier() might
>   	be required?
>   
>   Answer: Interestingly enough, rcu_barrier() was not originally
> @@ -292,7 +313,12 @@ Answer: Interestingly enough, rcu_barrier() was not originally
>   	implementing rcutorture, and found that rcu_barrier() solves
>   	this problem as well.
>   
> -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> +:ref:`Back to Quick Quiz #1 <rcubarrier_quiz_1>`
> +
> +.. _answer_rcubarrier_quiz_2:
> +
> +Quick Quiz #2:
> +	What happens if CPU 0's rcu_barrier_func() executes
>   	immediately (thus incrementing rcu_barrier_cpu_count to the
>   	value one), but the other CPU's rcu_barrier_func() invocations
>   	are delayed for a full grace period? Couldn't this result in
> @@ -323,3 +349,5 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last
>   	is to add an rcu_read_lock() before line 8 of rcu_barrier()
>   	and an rcu_read_unlock() after line 8 of this same function. If
>   	you can think of a better change, please let me know!
> +
> +:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
> 

Re: [Linux-kernel-mentees] [PATCH] Documentation: RCU: rcubarrier: Convert to reST

[Amol Grover]

On Thu, Nov 07, 2019 at 07:19:27AM +0700, Phong Tran wrote:
> On 11/6/19 11:56 PM, Amol Grover wrote:
> > Convert rcubarrier.txt to rcubarrier.rst and
> > add it to index.rst
> > 
> > Format file according to reST
> > - Add headings and sub-headings
> > - Add code segments
> > - Add cross-references to quizes and answers
> > 
> > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > ---
> >   Documentation/RCU/index.rst                   |   1 +
> >   .../RCU/{rcubarrier.txt => rcubarrier.rst}    | 220 ++++++++++--------
> >   2 files changed, 125 insertions(+), 96 deletions(-)
> >   rename Documentation/RCU/{rcubarrier.txt => rcubarrier.rst} (73%)
> > 
> > diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
> > index c81d0e4fd999..81a0a1e5f767 100644
> > --- a/Documentation/RCU/index.rst
> > +++ b/Documentation/RCU/index.rst
> > @@ -8,6 +8,7 @@ RCU concepts
> >      :maxdepth: 3
> >      arrayRCU
> > +   rcubarrier
> >      rcu_dereference
> >      whatisRCU
> >      rcu
> > diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.rst
> > similarity index 73%
> > rename from Documentation/RCU/rcubarrier.txt
> > rename to Documentation/RCU/rcubarrier.rst
> > index a2782df69732..1aa9ed1d1b5b 100644
> > --- a/Documentation/RCU/rcubarrier.txt
> > +++ b/Documentation/RCU/rcubarrier.rst
> > @@ -1,4 +1,7 @@
> > +.. _rcu_barrier:
> > +
> >   RCU and Unloadable Modules
> > +==========================
> >   [Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
> > @@ -21,7 +24,7 @@ given that readers might well leave absolutely no trace of their
> >   presence? There is a synchronize_rcu() primitive that blocks until all
> >   pre-existing readers have completed. An updater wishing to delete an
> >   element p from a linked list might do the following, while holding an
> > -appropriate lock, of course:
> > +appropriate lock, of course::
> >   	list_del_rcu(p);
> >   	synchronize_rcu();
> > @@ -32,13 +35,13 @@ primitive must be used instead. This primitive takes a pointer to an
> >   rcu_head struct placed within the RCU-protected data structure and
> >   another pointer to a function that may be invoked later to free that
> >   structure. Code to delete an element p from the linked list from IRQ
> > -context might then be as follows:
> > +context might then be as follows::
> >   	list_del_rcu(p);
> >   	call_rcu(&p->rcu, p_callback);
> >   Since call_rcu() never blocks, this code can safely be used from within
> > -IRQ context. The function p_callback() might be defined as follows:
> > +IRQ context. The function p_callback() might be defined as follows::
> >   	static void p_callback(struct rcu_head *rp)
> >   	{
> > @@ -49,6 +52,7 @@ IRQ context. The function p_callback() might be defined as follows:
> >   Unloading Modules That Use call_rcu()
> > +-------------------------------------
> >   But what if p_callback is defined in an unloadable module?
> > @@ -69,10 +73,11 @@ in realtime kernels in order to avoid excessive scheduling latencies.
> >   rcu_barrier()
> > +-------------
> >   We instead need the rcu_barrier() primitive.  Rather than waiting for
> >   a grace period to elapse, rcu_barrier() waits for all outstanding RCU
> > -callbacks to complete.  Please note that rcu_barrier() does -not- imply
> > +callbacks to complete.  Please note that rcu_barrier() does **not** imply
> >   synchronize_rcu(), in particular, if there are no RCU callbacks queued
> >   anywhere, rcu_barrier() is within its rights to return immediately,
> >   without waiting for a grace period to elapse.
> > @@ -89,78 +94,78 @@ module uses multiple flavors of call_rcu(), then it must also use multiple
> >   flavors of rcu_barrier() when unloading that module.  For example, if
> >   it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
> >   srcu_struct_2(), then the following three lines of code will be required
> 
> Hello Amol,
> 
> srcu_struct_2() should be srcu_struct_2

Hey Phong,
Thanks for the review! Fixed and sent the new patch
https://lore.kernel.org/lkml/20191107063241.GA2234@workstation-kernel-dev/

Thanks
Amol

> 
> > -when unloading:
> > +when unloading::
> >    1 rcu_barrier();
> >    2 srcu_barrier(&srcu_struct_1);
> >    3 srcu_barrier(&srcu_struct_2);
> >   The rcutorture module makes use of rcu_barrier() in its exit function
> > -as follows:
> > +as follows::
> > - 1 static void
> > - 2 rcu_torture_cleanup(void)
> > - 3 {
> > - 4   int i;
> > + 1  static void
> > + 2  rcu_torture_cleanup(void)
> > + 3  {
> > + 4    int i;
> >    5
> > - 6   fullstop = 1;
> > - 7   if (shuffler_task != NULL) {
> > + 6    fullstop = 1;
> > + 7    if (shuffler_task != NULL) {
> >    8     VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
> >    9     kthread_stop(shuffler_task);
> > -10   }
> > -11   shuffler_task = NULL;
> > -12
> > -13   if (writer_task != NULL) {
> > -14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> > -15     kthread_stop(writer_task);
> > -16   }
> > -17   writer_task = NULL;
> > -18
> > -19   if (reader_tasks != NULL) {
> > -20     for (i = 0; i < nrealreaders; i++) {
> > -21       if (reader_tasks[i] != NULL) {
> > -22         VERBOSE_PRINTK_STRING(
> > -23           "Stopping rcu_torture_reader task");
> > -24         kthread_stop(reader_tasks[i]);
> > -25       }
> > -26       reader_tasks[i] = NULL;
> > -27     }
> > -28     kfree(reader_tasks);
> > -29     reader_tasks = NULL;
> > -30   }
> > -31   rcu_torture_current = NULL;
> > -32
> > -33   if (fakewriter_tasks != NULL) {
> > -34     for (i = 0; i < nfakewriters; i++) {
> > -35       if (fakewriter_tasks[i] != NULL) {
> > -36         VERBOSE_PRINTK_STRING(
> > -37           "Stopping rcu_torture_fakewriter task");
> > -38         kthread_stop(fakewriter_tasks[i]);
> > -39       }
> > -40       fakewriter_tasks[i] = NULL;
> > -41     }
> > -42     kfree(fakewriter_tasks);
> > -43     fakewriter_tasks = NULL;
> > -44   }
> > -45
> > -46   if (stats_task != NULL) {
> > -47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> > -48     kthread_stop(stats_task);
> > -49   }
> > -50   stats_task = NULL;
> > -51
> > -52   /* Wait for all RCU callbacks to fire. */
> > -53   rcu_barrier();
> > -54
> > -55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> > -56
> > -57   if (cur_ops->cleanup != NULL)
> > -58     cur_ops->cleanup();
> > -59   if (atomic_read(&n_rcu_torture_error))
> > -60     rcu_torture_print_module_parms("End of test: FAILURE");
> > -61   else
> > -62     rcu_torture_print_module_parms("End of test: SUCCESS");
> > -63 }
> > + 10   }
> > + 11   shuffler_task = NULL;
> > + 12
> > + 13   if (writer_task != NULL) {
> > + 14     VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
> > + 15     kthread_stop(writer_task);
> > + 16   }
> > + 17   writer_task = NULL;
> > + 18
> > + 19   if (reader_tasks != NULL) {
> > + 20     for (i = 0; i < nrealreaders; i++) {
> > + 21       if (reader_tasks[i] != NULL) {
> > + 22         VERBOSE_PRINTK_STRING(
> > + 23           "Stopping rcu_torture_reader task");
> > + 24         kthread_stop(reader_tasks[i]);
> > + 25       }
> > + 26       reader_tasks[i] = NULL;
> > + 27     }
> > + 28     kfree(reader_tasks);
> > + 29     reader_tasks = NULL;
> > + 30   }
> > + 31   rcu_torture_current = NULL;
> > + 32
> > + 33   if (fakewriter_tasks != NULL) {
> > + 34     for (i = 0; i < nfakewriters; i++) {
> > + 35       if (fakewriter_tasks[i] != NULL) {
> > + 36         VERBOSE_PRINTK_STRING(
> > + 37           "Stopping rcu_torture_fakewriter task");
> > + 38         kthread_stop(fakewriter_tasks[i]);
> > + 39       }
> > + 40       fakewriter_tasks[i] = NULL;
> > + 41     }
> > + 42     kfree(fakewriter_tasks);
> > + 43     fakewriter_tasks = NULL;
> > + 44   }
> > + 45
> > + 46   if (stats_task != NULL) {
> > + 47     VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
> > + 48     kthread_stop(stats_task);
> > + 49   }
> > + 50   stats_task = NULL;
> > + 51
> > + 52   /* Wait for all RCU callbacks to fire. */
> > + 53   rcu_barrier();
> > + 54
> > + 55   rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
> > + 56
> > + 57   if (cur_ops->cleanup != NULL)
> > + 58     cur_ops->cleanup();
> > + 59   if (atomic_read(&n_rcu_torture_error))
> > + 60     rcu_torture_print_module_parms("End of test: FAILURE");
> > + 61   else
> > + 62     rcu_torture_print_module_parms("End of test: SUCCESS");
> > + 63 }
> >   Line 6 sets a global variable that prevents any RCU callbacks from
> >   re-posting themselves. This will not be necessary in most cases, since
> > @@ -176,9 +181,14 @@ for any pre-existing callbacks to complete.
> >   Then lines 55-62 print status and do operation-specific cleanup, and
> >   then return, permitting the module-unload operation to be completed.
> > -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> > +.. _rcubarrier_quiz_1:
> > +
> > +Quick Quiz #1:
> > +	Is there any other situation where rcu_barrier() might
> >   	be required?
> > +:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
> > +
> >   Your module might have additional complications. For example, if your
> >   module invokes call_rcu() from timers, you will need to first cancel all
> >   the timers, and only then invoke rcu_barrier() to wait for any remaining
> > @@ -188,11 +198,12 @@ Of course, if you module uses call_rcu(), you will need to invoke
> >   rcu_barrier() before unloading.  Similarly, if your module uses
> >   call_srcu(), you will need to invoke srcu_barrier() before unloading,
> >   and on the same srcu_struct structure.  If your module uses call_rcu()
> > --and- call_srcu(), then you will need to invoke rcu_barrier() -and-
> > +-and- call_srcu(), then you will need to invoke rcu_barrier() **and**
> 
> -and- here should be bold.
> 
> The rest looks good.
> 
> Regards,
> Phong.
> 
> >   srcu_barrier().
> >   Implementing rcu_barrier()
> > +--------------------------
> >   Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
> >   that RCU callbacks are never reordered once queued on one of the per-CPU
> > @@ -200,19 +211,19 @@ queues. His implementation queues an RCU callback on each of the per-CPU
> >   callback queues, and then waits until they have all started executing, at
> >   which point, all earlier RCU callbacks are guaranteed to have completed.
> > -The original code for rcu_barrier() was as follows:
> > +The original code for rcu_barrier() was as follows::
> > - 1 void rcu_barrier(void)
> > - 2 {
> > - 3   BUG_ON(in_interrupt());
> > - 4   /* Take cpucontrol mutex to protect against CPU hotplug */
> > - 5   mutex_lock(&rcu_barrier_mutex);
> > - 6   init_completion(&rcu_barrier_completion);
> > - 7   atomic_set(&rcu_barrier_cpu_count, 0);
> > - 8   on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> > - 9   wait_for_completion(&rcu_barrier_completion);
> > -10   mutex_unlock(&rcu_barrier_mutex);
> > -11 }
> > + 1  void rcu_barrier(void)
> > + 2  {
> > + 3    BUG_ON(in_interrupt());
> > + 4    /* Take cpucontrol mutex to protect against CPU hotplug */
> > + 5    mutex_lock(&rcu_barrier_mutex);
> > + 6    init_completion(&rcu_barrier_completion);
> > + 7    atomic_set(&rcu_barrier_cpu_count, 0);
> > + 8    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> > + 9    wait_for_completion(&rcu_barrier_completion);
> > + 10   mutex_unlock(&rcu_barrier_mutex);
> > + 11 }
> >   Line 3 verifies that the caller is in process context, and lines 5 and 10
> >   use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
> > @@ -226,18 +237,18 @@ This code was rewritten in 2008 and several times thereafter, but this
> >   still gives the general idea.
> >   The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
> > -to post an RCU callback, as follows:
> > +to post an RCU callback, as follows::
> > - 1 static void rcu_barrier_func(void *notused)
> > - 2 {
> > - 3 int cpu = smp_processor_id();
> > - 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> > - 5 struct rcu_head *head;
> > + 1  static void rcu_barrier_func(void *notused)
> > + 2  {
> > + 3    int cpu = smp_processor_id();
> > + 4    struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
> > + 5    struct rcu_head *head;
> >    6
> > - 7 head = &rdp->barrier;
> > - 8 atomic_inc(&rcu_barrier_cpu_count);
> > - 9 call_rcu(head, rcu_barrier_callback);
> > -10 }
> > + 7    head = &rdp->barrier;
> > + 8    atomic_inc(&rcu_barrier_cpu_count);
> > + 9    call_rcu(head, rcu_barrier_callback);
> > + 10 }
> >   Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
> >   which contains the struct rcu_head that needed for the later call to
> > @@ -248,20 +259,25 @@ the current CPU's queue.
> >   The rcu_barrier_callback() function simply atomically decrements the
> >   rcu_barrier_cpu_count variable and finalizes the completion when it
> > -reaches zero, as follows:
> > +reaches zero, as follows::
> >    1 static void rcu_barrier_callback(struct rcu_head *notused)
> >    2 {
> > - 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> > - 4 complete(&rcu_barrier_completion);
> > + 3   if (atomic_dec_and_test(&rcu_barrier_cpu_count))
> > + 4     complete(&rcu_barrier_completion);
> >    5 }
> > -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> > +.. _rcubarrier_quiz_2:
> > +
> > +Quick Quiz #2:
> > +	What happens if CPU 0's rcu_barrier_func() executes
> >   	immediately (thus incrementing rcu_barrier_cpu_count to the
> >   	value one), but the other CPU's rcu_barrier_func() invocations
> >   	are delayed for a full grace period? Couldn't this result in
> >   	rcu_barrier() returning prematurely?
> > +:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
> > +
> >   The current rcu_barrier() implementation is more complex, due to the need
> >   to avoid disturbing idle CPUs (especially on battery-powered systems)
> >   and the need to minimally disturb non-idle CPUs in real-time systems.
> > @@ -269,6 +285,7 @@ However, the code above illustrates the concepts.
> >   rcu_barrier() Summary
> > +---------------------
> >   The rcu_barrier() primitive has seen relatively little use, since most
> >   code using RCU is in the core kernel rather than in modules. However, if
> > @@ -277,8 +294,12 @@ so that your module may be safely unloaded.
> >   Answers to Quick Quizzes
> > +------------------------
> > +
> > +.. _answer_rcubarrier_quiz_1:
> > -Quick Quiz #1: Is there any other situation where rcu_barrier() might
> > +Quick Quiz #1:
> > +	Is there any other situation where rcu_barrier() might
> >   	be required?
> >   Answer: Interestingly enough, rcu_barrier() was not originally
> > @@ -292,7 +313,12 @@ Answer: Interestingly enough, rcu_barrier() was not originally
> >   	implementing rcutorture, and found that rcu_barrier() solves
> >   	this problem as well.
> > -Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
> > +:ref:`Back to Quick Quiz #1 <rcubarrier_quiz_1>`
> > +
> > +.. _answer_rcubarrier_quiz_2:
> > +
> > +Quick Quiz #2:
> > +	What happens if CPU 0's rcu_barrier_func() executes
> >   	immediately (thus incrementing rcu_barrier_cpu_count to the
> >   	value one), but the other CPU's rcu_barrier_func() invocations
> >   	are delayed for a full grace period? Couldn't this result in
> > @@ -323,3 +349,5 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last
> >   	is to add an rcu_read_lock() before line 8 of rcu_barrier()
> >   	and an rcu_read_unlock() after line 8 of this same function. If
> >   	you can think of a better change, please let me know!
> > +
> > +:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
> > 

Re: [Linux-kernel-mentees] [PATCH] Documentation: RCU: rcubarrier: Convert to reST

[Paul E. McKenney]

On Thu, Nov 07, 2019 at 12:09:49PM +0530, Amol Grover wrote:
> On Thu, Nov 07, 2019 at 07:19:27AM +0700, Phong Tran wrote:
> > On 11/6/19 11:56 PM, Amol Grover wrote:

[ . . . ]

> > >   We instead need the rcu_barrier() primitive.  Rather than waiting for
> > >   a grace period to elapse, rcu_barrier() waits for all outstanding RCU
> > > -callbacks to complete.  Please note that rcu_barrier() does -not- imply
> > > +callbacks to complete.  Please note that rcu_barrier() does **not** imply
> > >   synchronize_rcu(), in particular, if there are no RCU callbacks queued
> > >   anywhere, rcu_barrier() is within its rights to return immediately,
> > >   without waiting for a grace period to elapse.
> > > @@ -89,78 +94,78 @@ module uses multiple flavors of call_rcu(), then it must also use multiple
> > >   flavors of rcu_barrier() when unloading that module.  For example, if
> > >   it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
> > >   srcu_struct_2(), then the following three lines of code will be required
> > 
> > Hello Amol,
> > 
> > srcu_struct_2() should be srcu_struct_2
> 
> Hey Phong,
> Thanks for the review! Fixed and sent the new patch
> https://lore.kernel.org/lkml/20191107063241.GA2234@workstation-kernel-dev/

Phong, please let us know whether Amol's new version looks good to you.
If it does, preferably with your Reviewed-by and/or Tested by.  ;-)

							Thanx, Paul