Date:	Thu, 13 Nov 2008 18:11:52 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	corbet@....net
Subject: Re: [PATCH] rcupdate: move synchronize_sched() back to rcupdate.c
	V2

On Fri, Nov 14, 2008 at 09:03:20AM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > On Thu, Nov 13, 2008 at 10:48:33AM +0800, Lai Jiangshan wrote:
> >> Hi, Paul,
> >>
> >> Could you add a RCU document about unloadable modules for kernel?
> > 
> > You thinking in terms of an ASCII version of
> > http://lwn.net/Articles/217484/?
> > 
> > If so, please see attached patch and let me know what you think.
> > Being too lazy to convert the cartoon to ASCII graphics, I simply
> > left a URL to the .jpg on the LWN website.  Thus we need an ack/nack
> > from Jon Corbet (CCed).
> 
> Hi, Paul
> 
> Thank you. it's a very good document.

Updated version attached.

> I found several modules which need rcu_barrier(). So I'm going to
> do some cleanup for them. A document for rcu_barrier() will help
> these cleanup patches be accepted easily by maintainers.

Sounds very good!!!

> Lai.
> 
> > 
> > Of course, an alternative is to simply include the URL of the original
> > LWN article in 00-INDEX.  Thoughts?
> > 
> > 							Thanx, Paul
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> 
> > +The code for rcu_barrier() is 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 }
> > +
> 
> this is a little old.

Indeed!!!  Good catch!  I left this code, but added words saying that
it was the original and that it has since been rewritten to add
support for rcu_barrier_bh() and rcu_barrier_sched().

> > +
> > +Quick Quiz #2: Why is there no rcu_barrier_bh()?
> > +
> > +Answer: Because no one has needed it yet. As soon as someone needs to
> > +	use call_rcu_bh() from within an unloadable module, they will
> > +	need an rcu_barrier_bh().
> > +
> 
> add here.

Good point, I deleted this Quick Quiz.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

diff -urpNa -X dontdiff linux-2.6.27/Documentation/RCU/00-INDEX linux-2.6.27-rcu_barrierdoc/Documentation/RCU/00-INDEX
--- linux-2.6.27/Documentation/RCU/00-INDEX	2008-10-09 15:13:53.000000000 -0700
+++ linux-2.6.27-rcu_barrierdoc/Documentation/RCU/00-INDEX	2008-11-13 08:46:17.000000000 -0800
@@ -12,6 +12,8 @@ rcuref.txt
 	- Reference-count design for elements of lists/arrays protected by RCU
 rcu.txt
 	- RCU Concepts
+rcubarrier.txt
+	- Unloading modules that use RCU callbacks
 RTFP.txt
 	- List of RCU papers (bibliography) going back to 1980.
 torture.txt
diff -urpNa -X dontdiff linux-2.6.27/Documentation/RCU/rcubarrier.txt linux-2.6.27-rcu_barrierdoc/Documentation/RCU/rcubarrier.txt
--- linux-2.6.27/Documentation/RCU/rcubarrier.txt	1969-12-31 16:00:00.000000000 -0800
+++ linux-2.6.27-rcu_barrierdoc/Documentation/RCU/rcubarrier.txt	2008-11-13 18:08:35.000000000 -0800
@@ -0,0 +1,304 @@
+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. This primitive is similar
+to synchronize_rcu(), but instead of waiting solely for a grace
+period to elapse, it also waits for all outstanding RCU callbacks to
+complete. 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.
+
+Quick Quiz #1: Why is there no srcu_barrier()?
+
+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 #2: 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.
+
+
+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 to support rcu_barrier_bh() and
+rcu_barrier_sched() in addition to the original rcu_barrier().
+
+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 #3: 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?
+
+
+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: Why is there no srcu_barrier()?
+
+Answer: Since there is no call_srcu(), there can be no outstanding SRCU
+	callbacks. Therefore, there is no need to wait for them.
+
+Quick Quiz #2: 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 #3: 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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