354 lines
13 KiB
ReStructuredText
354 lines
13 KiB
ReStructuredText
.. _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_PREEMPTION
|
|
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_PREEMPTION 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_PREEMPTION 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>`
|