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Message-ID: <20191107063949.GA2310@workstation-kernel-dev>
Date: Thu, 7 Nov 2019 12:09:49 +0530
From: Amol Grover <frextrite@...il.com>
To: Phong Tran <tranmanphong@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>,
Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH] Documentation: RCU: rcubarrier:
Convert to reST
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@...il.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>`
> >
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