312 lines
12 KiB
Plaintext
312 lines
12 KiB
Plaintext
RCU and Unloadable Modules
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[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
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RCU (read-copy update) is a synchronization mechanism that can be thought
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of as a replacement for read-writer locking (among other things), but with
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very low-overhead readers that are immune to deadlock, priority inversion,
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and unbounded latency. RCU read-side critical sections are delimited
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by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT
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kernels, generate no code whatsoever.
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This means that RCU writers are unaware of the presence of concurrent
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readers, so that RCU updates to shared data must be undertaken quite
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carefully, leaving an old version of the data structure in place until all
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pre-existing readers have finished. These old versions are needed because
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such readers might hold a reference to them. RCU updates can therefore be
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rather expensive, and RCU is thus best suited for read-mostly situations.
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How can an RCU writer possibly determine when all readers are finished,
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given that readers might well leave absolutely no trace of their
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presence? There is a synchronize_rcu() primitive that blocks until all
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pre-existing readers have completed. An updater wishing to delete an
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element p from a linked list might do the following, while holding an
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appropriate lock, of course:
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list_del_rcu(p);
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synchronize_rcu();
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kfree(p);
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But the above code cannot be used in IRQ context -- the call_rcu()
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primitive must be used instead. This primitive takes a pointer to an
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rcu_head struct placed within the RCU-protected data structure and
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another pointer to a function that may be invoked later to free that
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structure. Code to delete an element p from the linked list from IRQ
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context might then be as follows:
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list_del_rcu(p);
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call_rcu(&p->rcu, p_callback);
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Since call_rcu() never blocks, this code can safely be used from within
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IRQ context. The function p_callback() might be defined as follows:
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static void p_callback(struct rcu_head *rp)
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{
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struct pstruct *p = container_of(rp, struct pstruct, rcu);
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kfree(p);
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}
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Unloading Modules That Use call_rcu()
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But what if p_callback is defined in an unloadable module?
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If we unload the module while some RCU callbacks are pending,
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the CPUs executing these callbacks are going to be severely
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disappointed when they are later invoked, as fancifully depicted at
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http://lwn.net/images/ns/kernel/rcu-drop.jpg.
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We could try placing a synchronize_rcu() in the module-exit code path,
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but this is not sufficient. Although synchronize_rcu() does wait for a
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grace period to elapse, it does not wait for the callbacks to complete.
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One might be tempted to try several back-to-back synchronize_rcu()
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calls, but this is still not guaranteed to work. If there is a very
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heavy RCU-callback load, then some of the callbacks might be deferred
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in order to allow other processing to proceed. Such deferral is required
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in realtime kernels in order to avoid excessive scheduling latencies.
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rcu_barrier()
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We instead need the rcu_barrier() primitive. This primitive is similar
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to synchronize_rcu(), but instead of waiting solely for a grace
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period to elapse, it also waits for all outstanding RCU callbacks to
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complete. Pseudo-code using rcu_barrier() is as follows:
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1. Prevent any new RCU callbacks from being posted.
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2. Execute rcu_barrier().
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3. Allow the module to be unloaded.
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Quick Quiz #1: Why is there no srcu_barrier()?
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The rcutorture module makes use of rcu_barrier in its exit function
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as follows:
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1 static void
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2 rcu_torture_cleanup(void)
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3 {
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4 int i;
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5
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6 fullstop = 1;
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7 if (shuffler_task != NULL) {
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8 VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
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9 kthread_stop(shuffler_task);
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10 }
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11 shuffler_task = NULL;
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12
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13 if (writer_task != NULL) {
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14 VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
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15 kthread_stop(writer_task);
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16 }
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17 writer_task = NULL;
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18
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19 if (reader_tasks != NULL) {
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20 for (i = 0; i < nrealreaders; i++) {
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21 if (reader_tasks[i] != NULL) {
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22 VERBOSE_PRINTK_STRING(
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23 "Stopping rcu_torture_reader task");
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24 kthread_stop(reader_tasks[i]);
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25 }
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26 reader_tasks[i] = NULL;
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27 }
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28 kfree(reader_tasks);
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29 reader_tasks = NULL;
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30 }
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31 rcu_torture_current = NULL;
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32
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33 if (fakewriter_tasks != NULL) {
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34 for (i = 0; i < nfakewriters; i++) {
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35 if (fakewriter_tasks[i] != NULL) {
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36 VERBOSE_PRINTK_STRING(
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37 "Stopping rcu_torture_fakewriter task");
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38 kthread_stop(fakewriter_tasks[i]);
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39 }
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40 fakewriter_tasks[i] = NULL;
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41 }
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42 kfree(fakewriter_tasks);
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43 fakewriter_tasks = NULL;
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44 }
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45
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46 if (stats_task != NULL) {
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47 VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task");
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48 kthread_stop(stats_task);
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49 }
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50 stats_task = NULL;
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51
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52 /* Wait for all RCU callbacks to fire. */
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53 rcu_barrier();
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54
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55 rcu_torture_stats_print(); /* -After- the stats thread is stopped! */
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56
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57 if (cur_ops->cleanup != NULL)
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58 cur_ops->cleanup();
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59 if (atomic_read(&n_rcu_torture_error))
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60 rcu_torture_print_module_parms("End of test: FAILURE");
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61 else
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62 rcu_torture_print_module_parms("End of test: SUCCESS");
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63 }
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Line 6 sets a global variable that prevents any RCU callbacks from
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re-posting themselves. This will not be necessary in most cases, since
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RCU callbacks rarely include calls to call_rcu(). However, the rcutorture
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module is an exception to this rule, and therefore needs to set this
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global variable.
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Lines 7-50 stop all the kernel tasks associated with the rcutorture
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module. Therefore, once execution reaches line 53, no more rcutorture
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RCU callbacks will be posted. The rcu_barrier() call on line 53 waits
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for any pre-existing callbacks to complete.
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Then lines 55-62 print status and do operation-specific cleanup, and
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then return, permitting the module-unload operation to be completed.
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Quick Quiz #2: Is there any other situation where rcu_barrier() might
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be required?
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Your module might have additional complications. For example, if your
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module invokes call_rcu() from timers, you will need to first cancel all
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the timers, and only then invoke rcu_barrier() to wait for any remaining
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RCU callbacks to complete.
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Of course, if you module uses call_rcu_bh(), you will need to invoke
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rcu_barrier_bh() before unloading. Similarly, if your module uses
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call_rcu_sched(), you will need to invoke rcu_barrier_sched() before
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unloading. If your module uses call_rcu(), call_rcu_bh(), -and-
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call_rcu_sched(), then you will need to invoke each of rcu_barrier(),
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rcu_barrier_bh(), and rcu_barrier_sched().
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Implementing rcu_barrier()
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Dipankar Sarma's implementation of rcu_barrier() makes use of the fact
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that RCU callbacks are never reordered once queued on one of the per-CPU
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queues. His implementation queues an RCU callback on each of the per-CPU
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callback queues, and then waits until they have all started executing, at
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which point, all earlier RCU callbacks are guaranteed to have completed.
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The original code for rcu_barrier() was as follows:
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1 void rcu_barrier(void)
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2 {
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3 BUG_ON(in_interrupt());
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4 /* Take cpucontrol mutex to protect against CPU hotplug */
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5 mutex_lock(&rcu_barrier_mutex);
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6 init_completion(&rcu_barrier_completion);
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7 atomic_set(&rcu_barrier_cpu_count, 0);
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8 on_each_cpu(rcu_barrier_func, NULL, 0, 1);
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9 wait_for_completion(&rcu_barrier_completion);
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10 mutex_unlock(&rcu_barrier_mutex);
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11 }
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Line 3 verifies that the caller is in process context, and lines 5 and 10
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use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
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global completion and counters at a time, which are initialized on lines
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6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is
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shown below. Note that the final "1" in on_each_cpu()'s argument list
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ensures that all the calls to rcu_barrier_func() will have completed
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before on_each_cpu() returns. Line 9 then waits for the completion.
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This code was rewritten in 2008 to support rcu_barrier_bh() and
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rcu_barrier_sched() in addition to the original rcu_barrier().
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The rcu_barrier_func() runs on each CPU, where it invokes call_rcu()
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to post an RCU callback, as follows:
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1 static void rcu_barrier_func(void *notused)
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2 {
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3 int cpu = smp_processor_id();
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4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
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5 struct rcu_head *head;
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6
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7 head = &rdp->barrier;
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8 atomic_inc(&rcu_barrier_cpu_count);
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9 call_rcu(head, rcu_barrier_callback);
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10 }
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Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
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which contains the struct rcu_head that needed for the later call to
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call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line
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8 increments a global counter. This counter will later be decremented
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by the callback. Line 9 then registers the rcu_barrier_callback() on
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the current CPU's queue.
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The rcu_barrier_callback() function simply atomically decrements the
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rcu_barrier_cpu_count variable and finalizes the completion when it
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reaches zero, as follows:
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1 static void rcu_barrier_callback(struct rcu_head *notused)
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2 {
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3 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
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4 complete(&rcu_barrier_completion);
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5 }
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Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
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immediately (thus incrementing rcu_barrier_cpu_count to the
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value one), but the other CPU's rcu_barrier_func() invocations
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are delayed for a full grace period? Couldn't this result in
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rcu_barrier() returning prematurely?
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rcu_barrier() Summary
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The rcu_barrier() primitive has seen relatively little use, since most
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code using RCU is in the core kernel rather than in modules. However, if
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you are using RCU from an unloadable module, you need to use rcu_barrier()
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so that your module may be safely unloaded.
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Answers to Quick Quizzes
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Quick Quiz #1: Why is there no srcu_barrier()?
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Answer: Since there is no call_srcu(), there can be no outstanding SRCU
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callbacks. Therefore, there is no need to wait for them.
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Quick Quiz #2: Is there any other situation where rcu_barrier() might
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be required?
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Answer: Interestingly enough, rcu_barrier() was not originally
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implemented for module unloading. Nikita Danilov was using
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RCU in a filesystem, which resulted in a similar situation at
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filesystem-unmount time. Dipankar Sarma coded up rcu_barrier()
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in response, so that Nikita could invoke it during the
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filesystem-unmount process.
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Much later, yours truly hit the RCU module-unload problem when
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implementing rcutorture, and found that rcu_barrier() solves
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this problem as well.
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Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
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immediately (thus incrementing rcu_barrier_cpu_count to the
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value one), but the other CPU's rcu_barrier_func() invocations
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are delayed for a full grace period? Couldn't this result in
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rcu_barrier() returning prematurely?
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Answer: This cannot happen. The reason is that on_each_cpu() has its last
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argument, the wait flag, set to "1". This flag is passed through
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to smp_call_function() and further to smp_call_function_on_cpu(),
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causing this latter to spin until the cross-CPU invocation of
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rcu_barrier_func() has completed. This by itself would prevent
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a grace period from completing on non-CONFIG_PREEMPT kernels,
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since each CPU must undergo a context switch (or other quiescent
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state) before the grace period can complete. However, this is
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of no use in CONFIG_PREEMPT kernels.
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Therefore, on_each_cpu() disables preemption across its call
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to smp_call_function() and also across the local call to
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rcu_barrier_func(). This prevents the local CPU from context
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switching, again preventing grace periods from completing. This
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means that all CPUs have executed rcu_barrier_func() before
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the first rcu_barrier_callback() can possibly execute, in turn
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preventing rcu_barrier_cpu_count from prematurely reaching zero.
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Currently, -rt implementations of RCU keep but a single global
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queue for RCU callbacks, and thus do not suffer from this
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problem. However, when the -rt RCU eventually does have per-CPU
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callback queues, things will have to change. One simple change
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is to add an rcu_read_lock() before line 8 of rcu_barrier()
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and an rcu_read_unlock() after line 8 of this same function. If
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you can think of a better change, please let me know!
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