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#if defined(CONFIG_MALLOC_TRIM)
35#include <malloc.h>
36#endif
37
38/*
39 * Global grace period counter. Bit 0 is always one in rcu_gp_ctr.
40 * Bits 1 and above are defined in synchronize_rcu.
41 */
42#define RCU_GP_LOCKED (1UL << 0)
43#define RCU_GP_CTR (1UL << 1)
44
45unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
46
47QemuEvent rcu_gp_event;
48static QemuMutex rcu_registry_lock;
49static QemuMutex rcu_sync_lock;
50
51/*
52 * Check whether a quiescent state was crossed between the beginning of
53 * update_counter_and_wait and now.
54 */
55static inline int rcu_gp_ongoing(unsigned long *ctr)
56{
57 unsigned long v;
58
59 v = atomic_read(ctr);
60 return v && (v != rcu_gp_ctr);
61}
62
63/* Written to only by each individual reader. Read by both the reader and the
64 * writers.
65 */
66__thread struct rcu_reader_data rcu_reader;
67
68/* Protected by rcu_registry_lock. */
69typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
70static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
71
72/* Wait for previous parity/grace period to be empty of readers. */
73static void wait_for_readers(void)
74{
75 ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders);
76 struct rcu_reader_data *index, *tmp;
77
78 for (;;) {
79 /* We want to be notified of changes made to rcu_gp_ongoing
80 * while we walk the list.
81 */
82 qemu_event_reset(&rcu_gp_event);
83
84 /* Instead of using atomic_mb_set for index->waiting, and
85 * atomic_mb_read for index->ctr, memory barriers are placed
86 * manually since writes to different threads are independent.
87 * qemu_event_reset has acquire semantics, so no memory barrier
88 * is needed here.
89 */
90 QLIST_FOREACH(index, &registry, node) {
91 atomic_set(&index->waiting, true);
92 }
93
94 /* Here, order the stores to index->waiting before the loads of
95 * index->ctr. Pairs with smp_mb_placeholder() in rcu_read_unlock(),
96 * ensuring that the loads of index->ctr are sequentially consistent.
97 */
98 smp_mb_global();
99
100 QLIST_FOREACH_SAFE(index, &registry, node, tmp) {
101 if (!rcu_gp_ongoing(&index->ctr)) {
102 QLIST_REMOVE(index, node);
103 QLIST_INSERT_HEAD(&qsreaders, index, node);
104
105 /* No need for mb_set here, worst of all we
106 * get some extra futex wakeups.
107 */
108 atomic_set(&index->waiting, false);
109 }
110 }
111
112 if (QLIST_EMPTY(&registry)) {
113 break;
114 }
115
116 /* Wait for one thread to report a quiescent state and try again.
117 * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
118 * wait too much time.
119 *
120 * rcu_register_thread() may add nodes to &registry; it will not
121 * wake up synchronize_rcu, but that is okay because at least another
122 * thread must exit its RCU read-side critical section before
123 * synchronize_rcu is done. The next iteration of the loop will
124 * move the new thread's rcu_reader from &registry to &qsreaders,
125 * because rcu_gp_ongoing() will return false.
126 *
127 * rcu_unregister_thread() may remove nodes from &qsreaders instead
128 * of &registry if it runs during qemu_event_wait. That's okay;
129 * the node then will not be added back to &registry by QLIST_SWAP
130 * below. The invariant is that the node is part of one list when
131 * rcu_registry_lock is released.
132 */
133 qemu_mutex_unlock(&rcu_registry_lock);
134 qemu_event_wait(&rcu_gp_event);
135 qemu_mutex_lock(&rcu_registry_lock);
136 }
137
138 /* put back the reader list in the registry */
139 QLIST_SWAP(&registry, &qsreaders, node);
140}
141
142void synchronize_rcu(void)
143{
144 qemu_mutex_lock(&rcu_sync_lock);
145
146 /* Write RCU-protected pointers before reading p_rcu_reader->ctr.
147 * Pairs with smp_mb_placeholder() in rcu_read_lock().
148 */
149 smp_mb_global();
150
151 qemu_mutex_lock(&rcu_registry_lock);
152 if (!QLIST_EMPTY(&registry)) {
153 /* In either case, the atomic_mb_set below blocks stores that free
154 * old RCU-protected pointers.
155 */
156 if (sizeof(rcu_gp_ctr) < 8) {
157 /* For architectures with 32-bit longs, a two-subphases algorithm
158 * ensures we do not encounter overflow bugs.
159 *
160 * Switch parity: 0 -> 1, 1 -> 0.
161 */
162 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
163 wait_for_readers();
164 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
165 } else {
166 /* Increment current grace period. */
167 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
168 }
169
170 wait_for_readers();
171 }
172
173 qemu_mutex_unlock(&rcu_registry_lock);
174 qemu_mutex_unlock(&rcu_sync_lock);
175}
176
177
178#define RCU_CALL_MIN_SIZE 30
179
180/* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
181 * from liburcu. Note that head is only used by the consumer.
182 */
183static struct rcu_head dummy;
184static struct rcu_head *head = &dummy, **tail = &dummy.next;
185static int rcu_call_count;
186static QemuEvent rcu_call_ready_event;
187
188static void enqueue(struct rcu_head *node)
189{
190 struct rcu_head **old_tail;
191
192 node->next = NULL;
193 old_tail = atomic_xchg(&tail, &node->next);
194 atomic_mb_set(old_tail, node);
195}
196
197static struct rcu_head *try_dequeue(void)
198{
199 struct rcu_head *node, *next;
200
201retry:
202 /* Test for an empty list, which we do not expect. Note that for
203 * the consumer head and tail are always consistent. The head
204 * is consistent because only the consumer reads/writes it.
205 * The tail, because it is the first step in the enqueuing.
206 * It is only the next pointers that might be inconsistent.
207 */
208 if (head == &dummy && atomic_mb_read(&tail) == &dummy.next) {
209 abort();
210 }
211
212 /* If the head node has NULL in its next pointer, the value is
213 * wrong and we need to wait until its enqueuer finishes the update.
214 */
215 node = head;
216 next = atomic_mb_read(&head->next);
217 if (!next) {
218 return NULL;
219 }
220
221 /* Since we are the sole consumer, and we excluded the empty case
222 * above, the queue will always have at least two nodes: the
223 * dummy node, and the one being removed. So we do not need to update
224 * the tail pointer.
225 */
226 head = next;
227
228 /* If we dequeued the dummy node, add it back at the end and retry. */
229 if (node == &dummy) {
230 enqueue(node);
231 goto retry;
232 }
233
234 return node;
235}
236
237static void *call_rcu_thread(void *opaque)
238{
239 struct rcu_head *node;
240
241 rcu_register_thread();
242
243 for (;;) {
244 int tries = 0;
245 int n = atomic_read(&rcu_call_count);
246
247 /* Heuristically wait for a decent number of callbacks to pile up.
248 * Fetch rcu_call_count now, we only must process elements that were
249 * added before synchronize_rcu() starts.
250 */
251 while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
252 g_usleep(10000);
253 if (n == 0) {
254 qemu_event_reset(&rcu_call_ready_event);
255 n = atomic_read(&rcu_call_count);
256 if (n == 0) {
257#if defined(CONFIG_MALLOC_TRIM)
258 malloc_trim(4 * 1024 * 1024);
259#endif
260 qemu_event_wait(&rcu_call_ready_event);
261 }
262 }
263 n = atomic_read(&rcu_call_count);
264 }
265
266 atomic_sub(&rcu_call_count, n);
267 synchronize_rcu();
268 qemu_mutex_lock_iothread();
269 while (n > 0) {
270 node = try_dequeue();
271 while (!node) {
272 qemu_mutex_unlock_iothread();
273 qemu_event_reset(&rcu_call_ready_event);
274 node = try_dequeue();
275 if (!node) {
276 qemu_event_wait(&rcu_call_ready_event);
277 node = try_dequeue();
278 }
279 qemu_mutex_lock_iothread();
280 }
281
282 n--;
283 node->func(node);
284 }
285 qemu_mutex_unlock_iothread();
286 }
287 abort();
288}
289
290void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
291{
292 node->func = func;
293 enqueue(node);
294 atomic_inc(&rcu_call_count);
295 qemu_event_set(&rcu_call_ready_event);
296}
297
298void rcu_register_thread(void)
299{
300 assert(rcu_reader.ctr == 0);
301 qemu_mutex_lock(&rcu_registry_lock);
302 QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
303 qemu_mutex_unlock(&rcu_registry_lock);
304}
305
306void rcu_unregister_thread(void)
307{
308 qemu_mutex_lock(&rcu_registry_lock);
309 QLIST_REMOVE(&rcu_reader, node);
310 qemu_mutex_unlock(&rcu_registry_lock);
311}
312
313static void rcu_init_complete(void)
314{
315 QemuThread thread;
316
317 qemu_mutex_init(&rcu_registry_lock);
318 qemu_mutex_init(&rcu_sync_lock);
319 qemu_event_init(&rcu_gp_event, true);
320
321 qemu_event_init(&rcu_call_ready_event, false);
322
323 /* The caller is assumed to have iothread lock, so the call_rcu thread
324 * must have been quiescent even after forking, just recreate it.
325 */
326 qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
327 NULL, QEMU_THREAD_DETACHED);
328
329 rcu_register_thread();
330}
331
332static int atfork_depth = 1;
333
334void rcu_enable_atfork(void)
335{
336 atfork_depth++;
337}
338
339void rcu_disable_atfork(void)
340{
341 atfork_depth--;
342}
343
344#ifdef CONFIG_POSIX
345static void rcu_init_lock(void)
346{
347 if (atfork_depth < 1) {
348 return;
349 }
350
351 qemu_mutex_lock(&rcu_sync_lock);
352 qemu_mutex_lock(&rcu_registry_lock);
353}
354
355static void rcu_init_unlock(void)
356{
357 if (atfork_depth < 1) {
358 return;
359 }
360
361 qemu_mutex_unlock(&rcu_registry_lock);
362 qemu_mutex_unlock(&rcu_sync_lock);
363}
364
365static void rcu_init_child(void)
366{
367 if (atfork_depth < 1) {
368 return;
369 }
370
371 memset(&registry, 0, sizeof(registry));
372 rcu_init_complete();
373}
374#endif
375
376static void __attribute__((__constructor__)) rcu_init(void)
377{
378 smp_mb_global_init();
379#ifdef CONFIG_POSIX
380 pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
381#endif
382 rcu_init_complete();
383}
384