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7911747b PB |
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 | ||
aafd7584 | 29 | #include "qemu/osdep.h" |
7911747b PB |
30 | #include "qemu/rcu.h" |
31 | #include "qemu/atomic.h" | |
26387f86 | 32 | #include "qemu/thread.h" |
a4649824 | 33 | #include "qemu/main-loop.h" |
6e8a355d | 34 | #include "qemu/lockable.h" |
5a22ab71 YZ |
35 | #if defined(CONFIG_MALLOC_TRIM) |
36 | #include <malloc.h> | |
37 | #endif | |
7911747b PB |
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; | |
ef149763 | 49 | static int in_drain_call_rcu; |
c097a60b WC |
50 | static QemuMutex rcu_registry_lock; |
51 | static QemuMutex rcu_sync_lock; | |
7911747b PB |
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 | ||
d73415a3 | 61 | v = qatomic_read(ctr); |
7911747b PB |
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 | */ | |
17c78154 | 68 | QEMU_DEFINE_CO_TLS(struct rcu_reader_data, rcu_reader) |
7911747b | 69 | |
c097a60b | 70 | /* Protected by rcu_registry_lock. */ |
7911747b PB |
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 | ||
7911747b | 86 | QLIST_FOREACH(index, ®istry, node) { |
d73415a3 | 87 | qatomic_set(&index->waiting, true); |
7911747b PB |
88 | } |
89 | ||
77a8b846 | 90 | /* Here, order the stores to index->waiting before the loads of |
c8d3877e | 91 | * index->ctr. Pairs with smp_mb_placeholder() in rcu_read_unlock(), |
77a8b846 | 92 | * ensuring that the loads of index->ctr are sequentially consistent. |
6e288b00 PB |
93 | * |
94 | * If this is the last iteration, this barrier also prevents | |
95 | * frees from seeping upwards, and orders the two wait phases | |
96 | * on architectures with 32-bit longs; see synchronize_rcu(). | |
e11131b0 | 97 | */ |
c8d3877e | 98 | smp_mb_global(); |
7911747b PB |
99 | |
100 | QLIST_FOREACH_SAFE(index, ®istry, node, tmp) { | |
101 | if (!rcu_gp_ongoing(&index->ctr)) { | |
102 | QLIST_REMOVE(index, node); | |
103 | QLIST_INSERT_HEAD(&qsreaders, index, node); | |
104 | ||
6e288b00 | 105 | /* No need for memory barriers here, worst of all we |
7911747b PB |
106 | * get some extra futex wakeups. |
107 | */ | |
d73415a3 | 108 | qatomic_set(&index->waiting, false); |
ef149763 GK |
109 | } else if (qatomic_read(&in_drain_call_rcu)) { |
110 | notifier_list_notify(&index->force_rcu, NULL); | |
7911747b PB |
111 | } |
112 | } | |
113 | ||
7911747b PB |
114 | if (QLIST_EMPTY(®istry)) { |
115 | break; | |
116 | } | |
117 | ||
c097a60b WC |
118 | /* Wait for one thread to report a quiescent state and try again. |
119 | * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't | |
120 | * wait too much time. | |
121 | * | |
122 | * rcu_register_thread() may add nodes to ®istry; it will not | |
123 | * wake up synchronize_rcu, but that is okay because at least another | |
124 | * thread must exit its RCU read-side critical section before | |
125 | * synchronize_rcu is done. The next iteration of the loop will | |
126 | * move the new thread's rcu_reader from ®istry to &qsreaders, | |
127 | * because rcu_gp_ongoing() will return false. | |
128 | * | |
129 | * rcu_unregister_thread() may remove nodes from &qsreaders instead | |
130 | * of ®istry if it runs during qemu_event_wait. That's okay; | |
131 | * the node then will not be added back to ®istry by QLIST_SWAP | |
132 | * below. The invariant is that the node is part of one list when | |
133 | * rcu_registry_lock is released. | |
7911747b | 134 | */ |
c097a60b | 135 | qemu_mutex_unlock(&rcu_registry_lock); |
7911747b | 136 | qemu_event_wait(&rcu_gp_event); |
c097a60b | 137 | qemu_mutex_lock(&rcu_registry_lock); |
7911747b PB |
138 | } |
139 | ||
140 | /* put back the reader list in the registry */ | |
141 | QLIST_SWAP(®istry, &qsreaders, node); | |
142 | } | |
143 | ||
144 | void synchronize_rcu(void) | |
145 | { | |
6e8a355d | 146 | QEMU_LOCK_GUARD(&rcu_sync_lock); |
7911747b | 147 | |
77a8b846 | 148 | /* Write RCU-protected pointers before reading p_rcu_reader->ctr. |
c8d3877e | 149 | * Pairs with smp_mb_placeholder() in rcu_read_lock(). |
6e288b00 PB |
150 | * |
151 | * Also orders write to RCU-protected pointers before | |
152 | * write to rcu_gp_ctr. | |
77a8b846 | 153 | */ |
c8d3877e | 154 | smp_mb_global(); |
77a8b846 | 155 | |
6e8a355d | 156 | QEMU_LOCK_GUARD(&rcu_registry_lock); |
7911747b | 157 | if (!QLIST_EMPTY(®istry)) { |
7911747b PB |
158 | if (sizeof(rcu_gp_ctr) < 8) { |
159 | /* For architectures with 32-bit longs, a two-subphases algorithm | |
160 | * ensures we do not encounter overflow bugs. | |
161 | * | |
162 | * Switch parity: 0 -> 1, 1 -> 0. | |
163 | */ | |
6e288b00 | 164 | qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); |
7911747b | 165 | wait_for_readers(); |
6e288b00 | 166 | qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); |
7911747b PB |
167 | } else { |
168 | /* Increment current grace period. */ | |
6e288b00 | 169 | qatomic_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); |
7911747b PB |
170 | } |
171 | ||
172 | wait_for_readers(); | |
173 | } | |
7911747b PB |
174 | } |
175 | ||
26387f86 PB |
176 | |
177 | #define RCU_CALL_MIN_SIZE 30 | |
178 | ||
179 | /* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h | |
180 | * from liburcu. Note that head is only used by the consumer. | |
181 | */ | |
182 | static struct rcu_head dummy; | |
183 | static struct rcu_head *head = &dummy, **tail = &dummy.next; | |
184 | static int rcu_call_count; | |
185 | static QemuEvent rcu_call_ready_event; | |
186 | ||
187 | static void enqueue(struct rcu_head *node) | |
188 | { | |
189 | struct rcu_head **old_tail; | |
190 | ||
191 | node->next = NULL; | |
8f593ba9 PB |
192 | |
193 | /* | |
194 | * Make this node the tail of the list. The node will be | |
195 | * used by further enqueue operations, but it will not | |
196 | * be dequeued yet... | |
197 | */ | |
d73415a3 | 198 | old_tail = qatomic_xchg(&tail, &node->next); |
8f593ba9 PB |
199 | |
200 | /* | |
201 | * ... until it is pointed to from another item in the list. | |
202 | * In the meantime, try_dequeue() will find a NULL next pointer | |
203 | * and loop. | |
204 | * | |
205 | * Synchronizes with qatomic_load_acquire() in try_dequeue(). | |
206 | */ | |
207 | qatomic_store_release(old_tail, node); | |
26387f86 PB |
208 | } |
209 | ||
210 | static struct rcu_head *try_dequeue(void) | |
211 | { | |
212 | struct rcu_head *node, *next; | |
213 | ||
214 | retry: | |
8f593ba9 PB |
215 | /* Head is only written by this thread, so no need for barriers. */ |
216 | node = head; | |
217 | ||
218 | /* | |
219 | * If the head node has NULL in its next pointer, the value is | |
220 | * wrong and we need to wait until its enqueuer finishes the update. | |
221 | */ | |
222 | next = qatomic_load_acquire(&node->next); | |
223 | if (!next) { | |
224 | return NULL; | |
225 | } | |
226 | ||
227 | /* | |
228 | * Test for an empty list, which we do not expect. Note that for | |
26387f86 PB |
229 | * the consumer head and tail are always consistent. The head |
230 | * is consistent because only the consumer reads/writes it. | |
231 | * The tail, because it is the first step in the enqueuing. | |
232 | * It is only the next pointers that might be inconsistent. | |
233 | */ | |
8f593ba9 | 234 | if (head == &dummy && qatomic_read(&tail) == &dummy.next) { |
26387f86 PB |
235 | abort(); |
236 | } | |
237 | ||
8f593ba9 PB |
238 | /* |
239 | * Since we are the sole consumer, and we excluded the empty case | |
26387f86 PB |
240 | * above, the queue will always have at least two nodes: the |
241 | * dummy node, and the one being removed. So we do not need to update | |
242 | * the tail pointer. | |
243 | */ | |
244 | head = next; | |
245 | ||
246 | /* If we dequeued the dummy node, add it back at the end and retry. */ | |
247 | if (node == &dummy) { | |
248 | enqueue(node); | |
249 | goto retry; | |
250 | } | |
251 | ||
252 | return node; | |
253 | } | |
254 | ||
255 | static void *call_rcu_thread(void *opaque) | |
256 | { | |
257 | struct rcu_head *node; | |
258 | ||
ab28bd23 PB |
259 | rcu_register_thread(); |
260 | ||
26387f86 PB |
261 | for (;;) { |
262 | int tries = 0; | |
d73415a3 | 263 | int n = qatomic_read(&rcu_call_count); |
26387f86 PB |
264 | |
265 | /* Heuristically wait for a decent number of callbacks to pile up. | |
266 | * Fetch rcu_call_count now, we only must process elements that were | |
267 | * added before synchronize_rcu() starts. | |
268 | */ | |
a7d1d636 PB |
269 | while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) { |
270 | g_usleep(10000); | |
271 | if (n == 0) { | |
272 | qemu_event_reset(&rcu_call_ready_event); | |
d73415a3 | 273 | n = qatomic_read(&rcu_call_count); |
a7d1d636 | 274 | if (n == 0) { |
5a22ab71 YZ |
275 | #if defined(CONFIG_MALLOC_TRIM) |
276 | malloc_trim(4 * 1024 * 1024); | |
277 | #endif | |
a7d1d636 PB |
278 | qemu_event_wait(&rcu_call_ready_event); |
279 | } | |
26387f86 | 280 | } |
d73415a3 | 281 | n = qatomic_read(&rcu_call_count); |
26387f86 PB |
282 | } |
283 | ||
d73415a3 | 284 | qatomic_sub(&rcu_call_count, n); |
26387f86 | 285 | synchronize_rcu(); |
a4649824 | 286 | qemu_mutex_lock_iothread(); |
26387f86 PB |
287 | while (n > 0) { |
288 | node = try_dequeue(); | |
289 | while (!node) { | |
a4649824 | 290 | qemu_mutex_unlock_iothread(); |
26387f86 PB |
291 | qemu_event_reset(&rcu_call_ready_event); |
292 | node = try_dequeue(); | |
293 | if (!node) { | |
294 | qemu_event_wait(&rcu_call_ready_event); | |
295 | node = try_dequeue(); | |
296 | } | |
a4649824 | 297 | qemu_mutex_lock_iothread(); |
26387f86 PB |
298 | } |
299 | ||
300 | n--; | |
301 | node->func(node); | |
302 | } | |
a4649824 | 303 | qemu_mutex_unlock_iothread(); |
26387f86 PB |
304 | } |
305 | abort(); | |
306 | } | |
307 | ||
308 | void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node)) | |
309 | { | |
310 | node->func = func; | |
311 | enqueue(node); | |
d73415a3 | 312 | qatomic_inc(&rcu_call_count); |
26387f86 PB |
313 | qemu_event_set(&rcu_call_ready_event); |
314 | } | |
315 | ||
d816614c ML |
316 | |
317 | struct rcu_drain { | |
318 | struct rcu_head rcu; | |
319 | QemuEvent drain_complete_event; | |
320 | }; | |
321 | ||
322 | static void drain_rcu_callback(struct rcu_head *node) | |
323 | { | |
324 | struct rcu_drain *event = (struct rcu_drain *)node; | |
325 | qemu_event_set(&event->drain_complete_event); | |
326 | } | |
327 | ||
328 | /* | |
329 | * This function ensures that all pending RCU callbacks | |
330 | * on the current thread are done executing | |
331 | ||
332 | * drops big qemu lock during the wait to allow RCU thread | |
333 | * to process the callbacks | |
334 | * | |
335 | */ | |
336 | ||
337 | void drain_call_rcu(void) | |
338 | { | |
339 | struct rcu_drain rcu_drain; | |
340 | bool locked = qemu_mutex_iothread_locked(); | |
341 | ||
342 | memset(&rcu_drain, 0, sizeof(struct rcu_drain)); | |
343 | qemu_event_init(&rcu_drain.drain_complete_event, false); | |
344 | ||
345 | if (locked) { | |
346 | qemu_mutex_unlock_iothread(); | |
347 | } | |
348 | ||
349 | ||
350 | /* | |
351 | * RCU callbacks are invoked in the same order as in which they | |
352 | * are registered, thus we can be sure that when 'drain_rcu_callback' | |
353 | * is called, all RCU callbacks that were registered on this thread | |
354 | * prior to calling this function are completed. | |
355 | * | |
356 | * Note that since we have only one global queue of the RCU callbacks, | |
357 | * we also end up waiting for most of RCU callbacks that were registered | |
d02d06f8 | 358 | * on the other threads, but this is a side effect that shouldn't be |
d816614c ML |
359 | * assumed. |
360 | */ | |
361 | ||
ef149763 | 362 | qatomic_inc(&in_drain_call_rcu); |
d816614c ML |
363 | call_rcu1(&rcu_drain.rcu, drain_rcu_callback); |
364 | qemu_event_wait(&rcu_drain.drain_complete_event); | |
ef149763 | 365 | qatomic_dec(&in_drain_call_rcu); |
d816614c ML |
366 | |
367 | if (locked) { | |
368 | qemu_mutex_lock_iothread(); | |
369 | } | |
370 | ||
371 | } | |
372 | ||
7911747b PB |
373 | void rcu_register_thread(void) |
374 | { | |
17c78154 | 375 | assert(get_ptr_rcu_reader()->ctr == 0); |
c097a60b | 376 | qemu_mutex_lock(&rcu_registry_lock); |
17c78154 | 377 | QLIST_INSERT_HEAD(®istry, get_ptr_rcu_reader(), node); |
c097a60b | 378 | qemu_mutex_unlock(&rcu_registry_lock); |
7911747b PB |
379 | } |
380 | ||
381 | void rcu_unregister_thread(void) | |
382 | { | |
c097a60b | 383 | qemu_mutex_lock(&rcu_registry_lock); |
17c78154 | 384 | QLIST_REMOVE(get_ptr_rcu_reader(), node); |
c097a60b | 385 | qemu_mutex_unlock(&rcu_registry_lock); |
7911747b PB |
386 | } |
387 | ||
ef149763 GK |
388 | void rcu_add_force_rcu_notifier(Notifier *n) |
389 | { | |
390 | qemu_mutex_lock(&rcu_registry_lock); | |
17c78154 | 391 | notifier_list_add(&get_ptr_rcu_reader()->force_rcu, n); |
ef149763 GK |
392 | qemu_mutex_unlock(&rcu_registry_lock); |
393 | } | |
394 | ||
395 | void rcu_remove_force_rcu_notifier(Notifier *n) | |
396 | { | |
397 | qemu_mutex_lock(&rcu_registry_lock); | |
398 | notifier_remove(n); | |
399 | qemu_mutex_unlock(&rcu_registry_lock); | |
400 | } | |
401 | ||
21b7cf9e | 402 | static void rcu_init_complete(void) |
7911747b | 403 | { |
26387f86 PB |
404 | QemuThread thread; |
405 | ||
c097a60b WC |
406 | qemu_mutex_init(&rcu_registry_lock); |
407 | qemu_mutex_init(&rcu_sync_lock); | |
7911747b | 408 | qemu_event_init(&rcu_gp_event, true); |
26387f86 PB |
409 | |
410 | qemu_event_init(&rcu_call_ready_event, false); | |
21b7cf9e PB |
411 | |
412 | /* The caller is assumed to have iothread lock, so the call_rcu thread | |
413 | * must have been quiescent even after forking, just recreate it. | |
414 | */ | |
26387f86 PB |
415 | qemu_thread_create(&thread, "call_rcu", call_rcu_thread, |
416 | NULL, QEMU_THREAD_DETACHED); | |
417 | ||
7911747b PB |
418 | rcu_register_thread(); |
419 | } | |
21b7cf9e | 420 | |
73c6e401 PB |
421 | static int atfork_depth = 1; |
422 | ||
423 | void rcu_enable_atfork(void) | |
424 | { | |
425 | atfork_depth++; | |
426 | } | |
427 | ||
428 | void rcu_disable_atfork(void) | |
429 | { | |
430 | atfork_depth--; | |
431 | } | |
432 | ||
21b7cf9e PB |
433 | #ifdef CONFIG_POSIX |
434 | static void rcu_init_lock(void) | |
435 | { | |
73c6e401 PB |
436 | if (atfork_depth < 1) { |
437 | return; | |
438 | } | |
439 | ||
c097a60b WC |
440 | qemu_mutex_lock(&rcu_sync_lock); |
441 | qemu_mutex_lock(&rcu_registry_lock); | |
21b7cf9e PB |
442 | } |
443 | ||
444 | static void rcu_init_unlock(void) | |
445 | { | |
73c6e401 PB |
446 | if (atfork_depth < 1) { |
447 | return; | |
448 | } | |
449 | ||
c097a60b WC |
450 | qemu_mutex_unlock(&rcu_registry_lock); |
451 | qemu_mutex_unlock(&rcu_sync_lock); | |
21b7cf9e PB |
452 | } |
453 | ||
2a96a552 | 454 | static void rcu_init_child(void) |
21b7cf9e | 455 | { |
2a96a552 PB |
456 | if (atfork_depth < 1) { |
457 | return; | |
458 | } | |
459 | ||
21b7cf9e PB |
460 | memset(®istry, 0, sizeof(registry)); |
461 | rcu_init_complete(); | |
462 | } | |
2a96a552 | 463 | #endif |
21b7cf9e PB |
464 | |
465 | static void __attribute__((__constructor__)) rcu_init(void) | |
466 | { | |
c8d3877e | 467 | smp_mb_global_init(); |
21b7cf9e | 468 | #ifdef CONFIG_POSIX |
2a96a552 | 469 | pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child); |
21b7cf9e PB |
470 | #endif |
471 | rcu_init_complete(); | |
472 | } |