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+RCU and Unloadable Modules
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+
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+[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
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+
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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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+
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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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+
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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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+
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+ list_del_rcu(p);
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+ synchronize_rcu();
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+ kfree(p);
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+
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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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+
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+ list_del_rcu(p);
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+ call_rcu(&p->rcu, p_callback);
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+
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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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+
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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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+
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+ kfree(p);
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+ }
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+
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+
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+Unloading Modules That Use call_rcu()
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+
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+But what if p_callback is defined in an unloadable module?
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+
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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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+
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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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+
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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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+
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+
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+rcu_barrier()
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+
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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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+
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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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+
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+Quick Quiz #1: Why is there no srcu_barrier()?
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+
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+Implementing rcu_barrier()
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+
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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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+
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+The original code for rcu_barrier() was as follows:
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+
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+rcu_barrier() Summary
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+
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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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+
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+
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+Answers to Quick Quizzes
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+
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+Quick Quiz #1: Why is there no srcu_barrier()?
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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