Merge tag 'pull-request-2022-09-28' of https://gitlab.com/thuth/qemu into staging
[qemu.git] / util / rcu.c
1 /*
2 * urcu-mb.c
3 *
4 * Userspace RCU library with explicit memory barriers
5 *
6 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
7 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
8 * Copyright 2015 Red Hat, Inc.
9 *
10 * Ported to QEMU by Paolo Bonzini <pbonzini@redhat.com>
11 *
12 * This library is free software; you can redistribute it and/or
13 * modify it under the terms of the GNU Lesser General Public
14 * License as published by the Free Software Foundation; either
15 * version 2.1 of the License, or (at your option) any later version.
16 *
17 * This library is distributed in the hope that it will be useful,
18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
20 * Lesser General Public License for more details.
21 *
22 * You should have received a copy of the GNU Lesser General Public
23 * License along with this library; if not, write to the Free Software
24 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
25 *
26 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
27 */
28
29 #include "qemu/osdep.h"
30 #include "qemu/rcu.h"
31 #include "qemu/atomic.h"
32 #include "qemu/thread.h"
33 #include "qemu/main-loop.h"
34 #include "qemu/lockable.h"
35 #if defined(CONFIG_MALLOC_TRIM)
36 #include <malloc.h>
37 #endif
38
39 /*
40 * Global grace period counter. Bit 0 is always one in rcu_gp_ctr.
41 * Bits 1 and above are defined in synchronize_rcu.
42 */
43 #define RCU_GP_LOCKED (1UL << 0)
44 #define RCU_GP_CTR (1UL << 1)
45
46 unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
47
48 QemuEvent rcu_gp_event;
49 static int in_drain_call_rcu;
50 static QemuMutex rcu_registry_lock;
51 static QemuMutex rcu_sync_lock;
52
53 /*
54 * Check whether a quiescent state was crossed between the beginning of
55 * update_counter_and_wait and now.
56 */
57 static inline int rcu_gp_ongoing(unsigned long *ctr)
58 {
59 unsigned long v;
60
61 v = qatomic_read(ctr);
62 return v && (v != rcu_gp_ctr);
63 }
64
65 /* Written to only by each individual reader. Read by both the reader and the
66 * writers.
67 */
68 QEMU_DEFINE_CO_TLS(struct rcu_reader_data, rcu_reader)
69
70 /* Protected by rcu_registry_lock. */
71 typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
72 static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
73
74 /* Wait for previous parity/grace period to be empty of readers. */
75 static void wait_for_readers(void)
76 {
77 ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders);
78 struct rcu_reader_data *index, *tmp;
79
80 for (;;) {
81 /* We want to be notified of changes made to rcu_gp_ongoing
82 * while we walk the list.
83 */
84 qemu_event_reset(&rcu_gp_event);
85
86 /* Instead of using qatomic_mb_set for index->waiting, and
87 * qatomic_mb_read for index->ctr, memory barriers are placed
88 * manually since writes to different threads are independent.
89 * qemu_event_reset has acquire semantics, so no memory barrier
90 * is needed here.
91 */
92 QLIST_FOREACH(index, &registry, node) {
93 qatomic_set(&index->waiting, true);
94 }
95
96 /* Here, order the stores to index->waiting before the loads of
97 * index->ctr. Pairs with smp_mb_placeholder() in rcu_read_unlock(),
98 * ensuring that the loads of index->ctr are sequentially consistent.
99 */
100 smp_mb_global();
101
102 QLIST_FOREACH_SAFE(index, &registry, node, tmp) {
103 if (!rcu_gp_ongoing(&index->ctr)) {
104 QLIST_REMOVE(index, node);
105 QLIST_INSERT_HEAD(&qsreaders, index, node);
106
107 /* No need for mb_set here, worst of all we
108 * get some extra futex wakeups.
109 */
110 qatomic_set(&index->waiting, false);
111 } else if (qatomic_read(&in_drain_call_rcu)) {
112 notifier_list_notify(&index->force_rcu, NULL);
113 }
114 }
115
116 if (QLIST_EMPTY(&registry)) {
117 break;
118 }
119
120 /* Wait for one thread to report a quiescent state and try again.
121 * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
122 * wait too much time.
123 *
124 * rcu_register_thread() may add nodes to &registry; it will not
125 * wake up synchronize_rcu, but that is okay because at least another
126 * thread must exit its RCU read-side critical section before
127 * synchronize_rcu is done. The next iteration of the loop will
128 * move the new thread's rcu_reader from &registry to &qsreaders,
129 * because rcu_gp_ongoing() will return false.
130 *
131 * rcu_unregister_thread() may remove nodes from &qsreaders instead
132 * of &registry if it runs during qemu_event_wait. That's okay;
133 * the node then will not be added back to &registry by QLIST_SWAP
134 * below. The invariant is that the node is part of one list when
135 * rcu_registry_lock is released.
136 */
137 qemu_mutex_unlock(&rcu_registry_lock);
138 qemu_event_wait(&rcu_gp_event);
139 qemu_mutex_lock(&rcu_registry_lock);
140 }
141
142 /* put back the reader list in the registry */
143 QLIST_SWAP(&registry, &qsreaders, node);
144 }
145
146 void synchronize_rcu(void)
147 {
148 QEMU_LOCK_GUARD(&rcu_sync_lock);
149
150 /* Write RCU-protected pointers before reading p_rcu_reader->ctr.
151 * Pairs with smp_mb_placeholder() in rcu_read_lock().
152 */
153 smp_mb_global();
154
155 QEMU_LOCK_GUARD(&rcu_registry_lock);
156 if (!QLIST_EMPTY(&registry)) {
157 /* In either case, the qatomic_mb_set below blocks stores that free
158 * old RCU-protected pointers.
159 */
160 if (sizeof(rcu_gp_ctr) < 8) {
161 /* For architectures with 32-bit longs, a two-subphases algorithm
162 * ensures we do not encounter overflow bugs.
163 *
164 * Switch parity: 0 -> 1, 1 -> 0.
165 */
166 qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
167 wait_for_readers();
168 qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
169 } else {
170 /* Increment current grace period. */
171 qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
172 }
173
174 wait_for_readers();
175 }
176 }
177
178
179 #define RCU_CALL_MIN_SIZE 30
180
181 /* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
182 * from liburcu. Note that head is only used by the consumer.
183 */
184 static struct rcu_head dummy;
185 static struct rcu_head *head = &dummy, **tail = &dummy.next;
186 static int rcu_call_count;
187 static QemuEvent rcu_call_ready_event;
188
189 static void enqueue(struct rcu_head *node)
190 {
191 struct rcu_head **old_tail;
192
193 node->next = NULL;
194 old_tail = qatomic_xchg(&tail, &node->next);
195 qatomic_mb_set(old_tail, node);
196 }
197
198 static struct rcu_head *try_dequeue(void)
199 {
200 struct rcu_head *node, *next;
201
202 retry:
203 /* Test for an empty list, which we do not expect. Note that for
204 * the consumer head and tail are always consistent. The head
205 * is consistent because only the consumer reads/writes it.
206 * The tail, because it is the first step in the enqueuing.
207 * It is only the next pointers that might be inconsistent.
208 */
209 if (head == &dummy && qatomic_mb_read(&tail) == &dummy.next) {
210 abort();
211 }
212
213 /* If the head node has NULL in its next pointer, the value is
214 * wrong and we need to wait until its enqueuer finishes the update.
215 */
216 node = head;
217 next = qatomic_mb_read(&head->next);
218 if (!next) {
219 return NULL;
220 }
221
222 /* Since we are the sole consumer, and we excluded the empty case
223 * above, the queue will always have at least two nodes: the
224 * dummy node, and the one being removed. So we do not need to update
225 * the tail pointer.
226 */
227 head = next;
228
229 /* If we dequeued the dummy node, add it back at the end and retry. */
230 if (node == &dummy) {
231 enqueue(node);
232 goto retry;
233 }
234
235 return node;
236 }
237
238 static void *call_rcu_thread(void *opaque)
239 {
240 struct rcu_head *node;
241
242 rcu_register_thread();
243
244 for (;;) {
245 int tries = 0;
246 int n = qatomic_read(&rcu_call_count);
247
248 /* Heuristically wait for a decent number of callbacks to pile up.
249 * Fetch rcu_call_count now, we only must process elements that were
250 * added before synchronize_rcu() starts.
251 */
252 while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
253 g_usleep(10000);
254 if (n == 0) {
255 qemu_event_reset(&rcu_call_ready_event);
256 n = qatomic_read(&rcu_call_count);
257 if (n == 0) {
258 #if defined(CONFIG_MALLOC_TRIM)
259 malloc_trim(4 * 1024 * 1024);
260 #endif
261 qemu_event_wait(&rcu_call_ready_event);
262 }
263 }
264 n = qatomic_read(&rcu_call_count);
265 }
266
267 qatomic_sub(&rcu_call_count, n);
268 synchronize_rcu();
269 qemu_mutex_lock_iothread();
270 while (n > 0) {
271 node = try_dequeue();
272 while (!node) {
273 qemu_mutex_unlock_iothread();
274 qemu_event_reset(&rcu_call_ready_event);
275 node = try_dequeue();
276 if (!node) {
277 qemu_event_wait(&rcu_call_ready_event);
278 node = try_dequeue();
279 }
280 qemu_mutex_lock_iothread();
281 }
282
283 n--;
284 node->func(node);
285 }
286 qemu_mutex_unlock_iothread();
287 }
288 abort();
289 }
290
291 void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
292 {
293 node->func = func;
294 enqueue(node);
295 qatomic_inc(&rcu_call_count);
296 qemu_event_set(&rcu_call_ready_event);
297 }
298
299
300 struct rcu_drain {
301 struct rcu_head rcu;
302 QemuEvent drain_complete_event;
303 };
304
305 static void drain_rcu_callback(struct rcu_head *node)
306 {
307 struct rcu_drain *event = (struct rcu_drain *)node;
308 qemu_event_set(&event->drain_complete_event);
309 }
310
311 /*
312 * This function ensures that all pending RCU callbacks
313 * on the current thread are done executing
314
315 * drops big qemu lock during the wait to allow RCU thread
316 * to process the callbacks
317 *
318 */
319
320 void drain_call_rcu(void)
321 {
322 struct rcu_drain rcu_drain;
323 bool locked = qemu_mutex_iothread_locked();
324
325 memset(&rcu_drain, 0, sizeof(struct rcu_drain));
326 qemu_event_init(&rcu_drain.drain_complete_event, false);
327
328 if (locked) {
329 qemu_mutex_unlock_iothread();
330 }
331
332
333 /*
334 * RCU callbacks are invoked in the same order as in which they
335 * are registered, thus we can be sure that when 'drain_rcu_callback'
336 * is called, all RCU callbacks that were registered on this thread
337 * prior to calling this function are completed.
338 *
339 * Note that since we have only one global queue of the RCU callbacks,
340 * we also end up waiting for most of RCU callbacks that were registered
341 * on the other threads, but this is a side effect that shoudn't be
342 * assumed.
343 */
344
345 qatomic_inc(&in_drain_call_rcu);
346 call_rcu1(&rcu_drain.rcu, drain_rcu_callback);
347 qemu_event_wait(&rcu_drain.drain_complete_event);
348 qatomic_dec(&in_drain_call_rcu);
349
350 if (locked) {
351 qemu_mutex_lock_iothread();
352 }
353
354 }
355
356 void rcu_register_thread(void)
357 {
358 assert(get_ptr_rcu_reader()->ctr == 0);
359 qemu_mutex_lock(&rcu_registry_lock);
360 QLIST_INSERT_HEAD(&registry, get_ptr_rcu_reader(), node);
361 qemu_mutex_unlock(&rcu_registry_lock);
362 }
363
364 void rcu_unregister_thread(void)
365 {
366 qemu_mutex_lock(&rcu_registry_lock);
367 QLIST_REMOVE(get_ptr_rcu_reader(), node);
368 qemu_mutex_unlock(&rcu_registry_lock);
369 }
370
371 void rcu_add_force_rcu_notifier(Notifier *n)
372 {
373 qemu_mutex_lock(&rcu_registry_lock);
374 notifier_list_add(&get_ptr_rcu_reader()->force_rcu, n);
375 qemu_mutex_unlock(&rcu_registry_lock);
376 }
377
378 void rcu_remove_force_rcu_notifier(Notifier *n)
379 {
380 qemu_mutex_lock(&rcu_registry_lock);
381 notifier_remove(n);
382 qemu_mutex_unlock(&rcu_registry_lock);
383 }
384
385 static void rcu_init_complete(void)
386 {
387 QemuThread thread;
388
389 qemu_mutex_init(&rcu_registry_lock);
390 qemu_mutex_init(&rcu_sync_lock);
391 qemu_event_init(&rcu_gp_event, true);
392
393 qemu_event_init(&rcu_call_ready_event, false);
394
395 /* The caller is assumed to have iothread lock, so the call_rcu thread
396 * must have been quiescent even after forking, just recreate it.
397 */
398 qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
399 NULL, QEMU_THREAD_DETACHED);
400
401 rcu_register_thread();
402 }
403
404 static int atfork_depth = 1;
405
406 void rcu_enable_atfork(void)
407 {
408 atfork_depth++;
409 }
410
411 void rcu_disable_atfork(void)
412 {
413 atfork_depth--;
414 }
415
416 #ifdef CONFIG_POSIX
417 static void rcu_init_lock(void)
418 {
419 if (atfork_depth < 1) {
420 return;
421 }
422
423 qemu_mutex_lock(&rcu_sync_lock);
424 qemu_mutex_lock(&rcu_registry_lock);
425 }
426
427 static void rcu_init_unlock(void)
428 {
429 if (atfork_depth < 1) {
430 return;
431 }
432
433 qemu_mutex_unlock(&rcu_registry_lock);
434 qemu_mutex_unlock(&rcu_sync_lock);
435 }
436
437 static void rcu_init_child(void)
438 {
439 if (atfork_depth < 1) {
440 return;
441 }
442
443 memset(&registry, 0, sizeof(registry));
444 rcu_init_complete();
445 }
446 #endif
447
448 static void __attribute__((__constructor__)) rcu_init(void)
449 {
450 smp_mb_global_init();
451 #ifdef CONFIG_POSIX
452 pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
453 #endif
454 rcu_init_complete();
455 }