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doc: Convert to rcubarrier.txt to ReST 

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>
Tested-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Amol Grover 2019-11-07 12:02:41 +05:30 committed by Paul E. McKenney
parent b00aedf978
commit 4af498306f
2 changed files with 126 additions and 97 deletions
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1
Documentation/RCU/index.rst
View File

@ -8,6 +8,7 @@ RCU concepts
:maxdepth: 3
arrayRCU
rcubarrier
rcu_dereference
whatisRCU
rcu

222
Documentation/RCU/rcubarrier.txt → Documentation/RCU/rcubarrier.rst
View File

@ -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.
@ -88,79 +93,79 @@ must match the flavor of rcu_barrier() with that of call_rcu(). If your
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:
srcu_struct_2, then the following three lines of code will be required
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
------------------------
Quick Quiz #1: Is there any other situation where rcu_barrier() might
.. _answer_rcubarrier_quiz_1:
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>`