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

From: Paul E. McKenney
Date: Wed Nov 06 2019 - 12:13:37 EST

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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@xxxxxxxxx>

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
>

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