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Diffstat (limited to 'Documentation/RCU/rcubarrier.txt')
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diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt new file mode 100644 index 0000000..909602d --- /dev/null +++ b/Documentation/RCU/rcubarrier.txt @@ -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! |
