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diff --git a/Documentation/RCU/rcubarrier.rst b/Documentation/RCU/rcubarrier.rst new file mode 100644 index 0000000000..6da7f66da2 --- /dev/null +++ b/Documentation/RCU/rcubarrier.rst @@ -0,0 +1,380 @@ +.. _rcu_barrier: + +RCU and Unloadable Modules +========================== + +[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/] + +RCU updaters sometimes use call_rcu() to initiate an asynchronous wait for +a grace period to elapse. 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 the p_callback() function 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. For but one example, such +deferral is required in realtime kernels in order to avoid excessive +scheduling latencies. + + +rcu_barrier() +------------- + +This situation can be handled by 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 anything, let alone a grace period. + +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 srcu_barrier() with that of call_srcu(). +If your module uses multiple srcu_struct structures, then it must also +use multiple invocations of srcu_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); + +If latency is of the essence, workqueues could be used to run these +three functions concurrently. + +An ancient version of 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 refrain +from posting new timers, cancel (or wait for) all the already-posted +timers, and only then invoke rcu_barrier() to wait for any remaining +RCU callbacks to complete. + +Of course, if your 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 (as noted above) 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 roughly 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, 1); + 8 on_each_cpu(rcu_barrier_func, NULL, 0, 1); + 9 if (atomic_dec_and_test(&rcu_barrier_cpu_count)) + 10 complete(&rcu_barrier_completion); + 11 wait_for_completion(&rcu_barrier_completion); + 12 mutex_unlock(&rcu_barrier_mutex); + 13 } + +Line 3 verifies that the caller is in process context, and lines 5 and 12 +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 removes the initial count from +rcu_barrier_cpu_count, and if this count is now zero, line 10 finalizes +the completion, which prevents line 11 from blocking. Either way, +line 11 then waits (if needed) for the completion. + +.. _rcubarrier_quiz_2: + +Quick Quiz #2: + Why doesn't line 8 initialize rcu_barrier_cpu_count to zero, + thereby avoiding the need for lines 9 and 10? + +:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>` + +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 the 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_3: + +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? + +:ref:`Answer to Quick Quiz #3 <answer_rcubarrier_quiz_3>` + +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. +In addition, a great many optimizations have been applied. However, +the code above illustrates the concepts. + + +rcu_barrier() Summary +--------------------- + +The rcu_barrier() primitive is used relatively infrequently, 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: + Why doesn't line 8 initialize rcu_barrier_cpu_count to zero, + thereby avoiding the need for lines 9 and 10? + +Answer: + Suppose that the on_each_cpu() function shown on line 8 was + delayed, so that CPU 0's rcu_barrier_func() executed and + the corresponding grace period elapsed, all before CPU 1's + rcu_barrier_func() started executing. This would result in + rcu_barrier_cpu_count being decremented to zero, so that line + 11's wait_for_completion() would return immediately, failing to + wait for CPU 1's callbacks to be invoked. + + Note that this was not a problem when the rcu_barrier() code + was first added back in 2005. This is because on_each_cpu() + disables preemption, which acted as an RCU read-side critical + section, thus preventing CPU 0's grace period from completing + until on_each_cpu() had dealt with all of the CPUs. However, + with the advent of preemptible RCU, rcu_barrier() no longer + waited on nonpreemptible regions of code in preemptible kernels, + that being the job of the new rcu_barrier_sched() function. + + However, with the RCU flavor consolidation around v4.20, this + possibility was once again ruled out, because the consolidated + RCU once again waits on nonpreemptible regions of code. + + Nevertheless, that extra count might still be a good idea. + Relying on these sort of accidents of implementation can result + in later surprise bugs when the implementation changes. + +:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>` + +.. _answer_rcubarrier_quiz_3: + +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_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(). Because recent RCU implementations treat + preemption-disabled regions of code as RCU read-side critical + sections, this prevents 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. + + But if on_each_cpu() ever decides to forgo disabling preemption, + as might well happen due to real-time latency considerations, + initializing rcu_barrier_cpu_count to one will save the day. + +:ref:`Back to Quick Quiz #3 <rcubarrier_quiz_3>` |