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.

  Of course, if you module uses call_rcu_bh(), you will need to invoke
  rcu_barrier_bh() before unloading.  Similarly, if your module uses
  call_rcu_sched(), you will need to invoke rcu_barrier_sched() before
  unloading.  If your module uses call_rcu(), call_rcu_bh(), -and-
  call_rcu_sched(), then you will need to invoke each of rcu_barrier(),
  rcu_barrier_bh(), and rcu_barrier_sched().

  
  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!