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Date: Sat, 25 Jan 2020 10:42:49 -0000
From: "tip-bot2 for Amol Grover" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Amol Grover <frextrite@...il.com>,
Phong Tran <tranmanphong@...il.com>,
"Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] doc: Convert to rcubarrier.txt to ReST
The following commit has been merged into the core/rcu branch of tip:
Commit-ID: 4af498306ffd8e29ed5c1ae544d01bc8c09c3f8e
Gitweb: https://git.kernel.org/tip/4af498306ffd8e29ed5c1ae544d01bc8c09c3f8e
Author: Amol Grover <frextrite@...il.com>
AuthorDate: Thu, 07 Nov 2019 12:02:41 +05:30
Committer: Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Tue, 10 Dec 2019 18:51:53 -08:00
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@...il.com>
Tested-by: Phong Tran <tranmanphong@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
Documentation/RCU/index.rst | 1 +-
Documentation/RCU/rcubarrier.rst | 353 ++++++++++++++++++++++++++++++-
Documentation/RCU/rcubarrier.txt | 325 +----------------------------
3 files changed, 354 insertions(+), 325 deletions(-)
create mode 100644 Documentation/RCU/rcubarrier.rst
delete mode 100644 Documentation/RCU/rcubarrier.txt
diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index c81d0e4..81a0a1e 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.rst b/Documentation/RCU/rcubarrier.rst
new file mode 100644
index 0000000..f64f441
--- /dev/null
+++ b/Documentation/RCU/rcubarrier.rst
@@ -0,0 +1,353 @@
+.. _rcu_barrier:
+
+RCU and Unloadable Modules
+==========================
+
+[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
+
+RCU (read-copy update) is a synchronization mechanism that can be thought
+of as a replacement for read-writer locking (among other things), but with
+very low-overhead readers that are immune to deadlock, priority inversion,
+and unbounded latency. RCU read-side critical sections are delimited
+by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT
+kernels, generate no code whatsoever.
+
+This means that RCU writers are unaware of the presence of concurrent
+readers, so that RCU updates to shared data must be undertaken quite
+carefully, leaving an old version of the data structure in place until all
+pre-existing readers have finished. These old versions are needed because
+such readers might hold a reference to them. RCU updates can therefore be
+rather expensive, and RCU is thus best suited for read-mostly situations.
+
+How can an RCU writer possibly determine when all readers are finished,
+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::
+
+ list_del_rcu(p);
+ synchronize_rcu();
+ kfree(p);
+
+But the above code cannot be used in IRQ context -- the call_rcu()
+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::
+
+ 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::
+
+ static void p_callback(struct rcu_head *rp)
+ {
+ struct pstruct *p = container_of(rp, struct pstruct, rcu);
+
+ kfree(p);
+ }
+
+
+Unloading Modules That Use call_rcu()
+-------------------------------------
+
+But what if p_callback is defined in an unloadable module?
+
+If we unload the module while some RCU callbacks are pending,
+the CPUs executing these callbacks are going to be severely
+disappointed when they are later invoked, as fancifully depicted at
+http://lwn.net/images/ns/kernel/rcu-drop.jpg.
+
+We could try placing a synchronize_rcu() in the module-exit code path,
+but this is not sufficient. Although synchronize_rcu() does wait for a
+grace period to elapse, it does not wait for the callbacks to complete.
+
+One might be tempted to try several back-to-back synchronize_rcu()
+calls, but this is still not guaranteed to work. If there is a very
+heavy RCU-callback load, then some of the callbacks might be deferred
+in order to allow other processing to proceed. Such deferral is required
+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
+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.
+
+Pseudo-code using rcu_barrier() is as follows:
+
+ 1. Prevent any new RCU callbacks from being posted.
+ 2. Execute rcu_barrier().
+ 3. Allow the module to be unloaded.
+
+There is also an srcu_barrier() function for SRCU, and you of course
+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::
+
+ 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::
+
+ 1 static void
+ 2 rcu_torture_cleanup(void)
+ 3 {
+ 4 int i;
+ 5
+ 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 }
+
+Line 6 sets a global variable that prevents any RCU callbacks from
+re-posting themselves. This will not be necessary in most cases, since
+RCU callbacks rarely include calls to call_rcu(). However, the rcutorture
+module is an exception to this rule, and therefore needs to set this
+global variable.
+
+Lines 7-50 stop all the kernel tasks associated with the rcutorture
+module. Therefore, once execution reaches line 53, no more rcutorture
+RCU callbacks will be posted. The rcu_barrier() call on line 53 waits
+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.
+
+.. _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
+RCU callbacks to complete.
+
+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**
+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
+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::
+
+ 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
+global completion and counters at a time, which are initialized on lines
+6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is
+shown below. Note that the final "1" in on_each_cpu()'s argument list
+ensures that all the calls to rcu_barrier_func() will have completed
+before on_each_cpu() returns. Line 9 then waits for the completion.
+
+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::
+
+ 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 }
+
+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
+call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line
+8 increments a global counter. This counter will later be decremented
+by the callback. Line 9 then registers the rcu_barrier_callback() on
+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::
+
+ 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);
+ 5 }
+
+.. _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.
+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
+you are using RCU from an unloadable module, you need to use rcu_barrier()
+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
+ be required?
+
+Answer: Interestingly enough, rcu_barrier() was not originally
+ implemented for module unloading. Nikita Danilov was using
+ RCU in a filesystem, which resulted in a similar situation at
+ filesystem-unmount time. Dipankar Sarma coded up rcu_barrier()
+ in response, so that Nikita could invoke it during the
+ filesystem-unmount process.
+
+ Much later, yours truly hit the RCU module-unload problem when
+ implementing rcutorture, and found that rcu_barrier() solves
+ this problem as well.
+
+: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
+ rcu_barrier() returning prematurely?
+
+Answer: This cannot happen. The reason is that on_each_cpu() has its last
+ argument, the wait flag, set to "1". This flag is passed through
+ to smp_call_function() and further to smp_call_function_on_cpu(),
+ causing this latter to spin until the cross-CPU invocation of
+ rcu_barrier_func() has completed. This by itself would prevent
+ a grace period from completing on non-CONFIG_PREEMPT kernels,
+ since each CPU must undergo a context switch (or other quiescent
+ state) before the grace period can complete. However, this is
+ of no use in CONFIG_PREEMPT kernels.
+
+ Therefore, on_each_cpu() disables preemption across its call
+ to smp_call_function() and also across the local call to
+ rcu_barrier_func(). This prevents the local CPU from context
+ switching, again preventing grace periods from completing. This
+ means that all CPUs have executed rcu_barrier_func() before
+ the first rcu_barrier_callback() can possibly execute, in turn
+ preventing rcu_barrier_cpu_count from prematurely reaching zero.
+
+ Currently, -rt implementations of RCU keep but a single global
+ queue for RCU callbacks, and thus do not suffer from this
+ problem. However, when the -rt RCU eventually does have per-CPU
+ callback queues, things will have to change. One simple change
+ 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>`
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
deleted file mode 100644
index a2782df..0000000
--- a/Documentation/RCU/rcubarrier.txt
+++ /dev/null
@@ -1,325 +0,0 @@
-RCU and Unloadable Modules
-
-[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
-
-RCU (read-copy update) is a synchronization mechanism that can be thought
-of as a replacement for read-writer locking (among other things), but with
-very low-overhead readers that are immune to deadlock, priority inversion,
-and unbounded latency. RCU read-side critical sections are delimited
-by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT
-kernels, generate no code whatsoever.
-
-This means that RCU writers are unaware of the presence of concurrent
-readers, so that RCU updates to shared data must be undertaken quite
-carefully, leaving an old version of the data structure in place until all
-pre-existing readers have finished. These old versions are needed because
-such readers might hold a reference to them. RCU updates can therefore be
-rather expensive, and RCU is thus best suited for read-mostly situations.
-
-How can an RCU writer possibly determine when all readers are finished,
-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:
-
- list_del_rcu(p);
- synchronize_rcu();
- kfree(p);
-
-But the above code cannot be used in IRQ context -- the call_rcu()
-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:
-
- 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:
-
- static void p_callback(struct rcu_head *rp)
- {
- struct pstruct *p = container_of(rp, struct pstruct, rcu);
-
- kfree(p);
- }
-
-
-Unloading Modules That Use call_rcu()
-
-But what if p_callback is defined in an unloadable module?
-
-If we unload the module while some RCU callbacks are pending,
-the CPUs executing these callbacks are going to be severely
-disappointed when they are later invoked, as fancifully depicted at
-http://lwn.net/images/ns/kernel/rcu-drop.jpg.
-
-We could try placing a synchronize_rcu() in the module-exit code path,
-but this is not sufficient. Although synchronize_rcu() does wait for a
-grace period to elapse, it does not wait for the callbacks to complete.
-
-One might be tempted to try several back-to-back synchronize_rcu()
-calls, but this is still not guaranteed to work. If there is a very
-heavy RCU-callback load, then some of the callbacks might be deferred
-in order to allow other processing to proceed. Such deferral is required
-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
-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.
-
-Pseudo-code using rcu_barrier() is as follows:
-
- 1. Prevent any new RCU callbacks from being posted.
- 2. Execute rcu_barrier().
- 3. Allow the module to be unloaded.
-
-There is also an srcu_barrier() function for SRCU, and you of course
-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:
-
- 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:
-
- 1 static void
- 2 rcu_torture_cleanup(void)
- 3 {
- 4 int i;
- 5
- 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 }
-
-Line 6 sets a global variable that prevents any RCU callbacks from
-re-posting themselves. This will not be necessary in most cases, since
-RCU callbacks rarely include calls to call_rcu(). However, the rcutorture
-module is an exception to this rule, and therefore needs to set this
-global variable.
-
-Lines 7-50 stop all the kernel tasks associated with the rcutorture
-module. Therefore, once execution reaches line 53, no more rcutorture
-RCU callbacks will be posted. The rcu_barrier() call on line 53 waits
-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
- be required?
-
-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
-RCU callbacks to complete.
-
-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-
-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
-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:
-
- 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
-global completion and counters at a time, which are initialized on lines
-6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is
-shown below. Note that the final "1" in on_each_cpu()'s argument list
-ensures that all the calls to rcu_barrier_func() will have completed
-before on_each_cpu() returns. Line 9 then waits for the completion.
-
-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:
-
- 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 }
-
-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
-call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line
-8 increments a global counter. This counter will later be decremented
-by the callback. Line 9 then registers the rcu_barrier_callback() on
-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:
-
- 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);
- 5 }
-
-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?
-
-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.
-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
-you are using RCU from an unloadable module, you need to use rcu_barrier()
-so that your module may be safely unloaded.
-
-
-Answers to Quick Quizzes
-
-Quick Quiz #1: Is there any other situation where rcu_barrier() might
- be required?
-
-Answer: Interestingly enough, rcu_barrier() was not originally
- implemented for module unloading. Nikita Danilov was using
- RCU in a filesystem, which resulted in a similar situation at
- filesystem-unmount time. Dipankar Sarma coded up rcu_barrier()
- in response, so that Nikita could invoke it during the
- filesystem-unmount process.
-
- Much later, yours truly hit the RCU module-unload problem when
- 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
- 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?
-
-Answer: This cannot happen. The reason is that on_each_cpu() has its last
- argument, the wait flag, set to "1". This flag is passed through
- to smp_call_function() and further to smp_call_function_on_cpu(),
- causing this latter to spin until the cross-CPU invocation of
- rcu_barrier_func() has completed. This by itself would prevent
- a grace period from completing on non-CONFIG_PREEMPT kernels,
- since each CPU must undergo a context switch (or other quiescent
- state) before the grace period can complete. However, this is
- of no use in CONFIG_PREEMPT kernels.
-
- Therefore, on_each_cpu() disables preemption across its call
- to smp_call_function() and also across the local call to
- rcu_barrier_func(). This prevents the local CPU from context
- switching, again preventing grace periods from completing. This
- means that all CPUs have executed rcu_barrier_func() before
- the first rcu_barrier_callback() can possibly execute, in turn
- preventing rcu_barrier_cpu_count from prematurely reaching zero.
-
- Currently, -rt implementations of RCU keep but a single global
- queue for RCU callbacks, and thus do not suffer from this
- problem. However, when the -rt RCU eventually does have per-CPU
- callback queues, things will have to change. One simple change
- 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!
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