[Linux-kernel-mentees] [PATCH] Documentation: RCU: rcubarrier: Convert to reST
Amol Grover
frextrite at gmail.com
Wed Nov 6 16:56:17 UTC 2019
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 at 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
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