Merge remote-tracking branch 'remotes/kraxel/tags/usb-20180126-v3-pull-request' into...
[qemu/ar7.git] / util / rcu.c
blobf4d09c83044666013a6b44fb571621854f3b0b86
1 /*
2 * urcu-mb.c
4 * Userspace RCU library with explicit memory barriers
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.
10 * Ported to QEMU by Paolo Bonzini <pbonzini@redhat.com>
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.
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.
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
26 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
29 #include "qemu/osdep.h"
30 #include "qemu-common.h"
31 #include "qemu/rcu.h"
32 #include "qemu/atomic.h"
33 #include "qemu/thread.h"
34 #include "qemu/main-loop.h"
35 #if defined(CONFIG_MALLOC_TRIM)
36 #include <malloc.h>
37 #endif
40 * Global grace period counter. Bit 0 is always one in rcu_gp_ctr.
41 * Bits 1 and above are defined in synchronize_rcu.
43 #define RCU_GP_LOCKED (1UL << 0)
44 #define RCU_GP_CTR (1UL << 1)
46 unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
48 QemuEvent rcu_gp_event;
49 static QemuMutex rcu_registry_lock;
50 static QemuMutex rcu_sync_lock;
53 * Check whether a quiescent state was crossed between the beginning of
54 * update_counter_and_wait and now.
56 static inline int rcu_gp_ongoing(unsigned long *ctr)
58 unsigned long v;
60 v = atomic_read(ctr);
61 return v && (v != rcu_gp_ctr);
64 /* Written to only by each individual reader. Read by both the reader and the
65 * writers.
67 __thread struct rcu_reader_data rcu_reader;
69 /* Protected by rcu_registry_lock. */
70 typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
71 static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
73 /* Wait for previous parity/grace period to be empty of readers. */
74 static void wait_for_readers(void)
76 ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders);
77 struct rcu_reader_data *index, *tmp;
79 for (;;) {
80 /* We want to be notified of changes made to rcu_gp_ongoing
81 * while we walk the list.
83 qemu_event_reset(&rcu_gp_event);
85 /* Instead of using atomic_mb_set for index->waiting, and
86 * atomic_mb_read for index->ctr, memory barriers are placed
87 * manually since writes to different threads are independent.
88 * qemu_event_reset has acquire semantics, so no memory barrier
89 * is needed here.
91 QLIST_FOREACH(index, &registry, node) {
92 atomic_set(&index->waiting, true);
95 /* Here, order the stores to index->waiting before the
96 * loads of index->ctr.
98 smp_mb();
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);
105 /* No need for mb_set here, worst of all we
106 * get some extra futex wakeups.
108 atomic_set(&index->waiting, false);
112 if (QLIST_EMPTY(&registry)) {
113 break;
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.
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.
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.
133 qemu_mutex_unlock(&rcu_registry_lock);
134 qemu_event_wait(&rcu_gp_event);
135 qemu_mutex_lock(&rcu_registry_lock);
138 /* put back the reader list in the registry */
139 QLIST_SWAP(&registry, &qsreaders, node);
142 void synchronize_rcu(void)
144 qemu_mutex_lock(&rcu_sync_lock);
145 qemu_mutex_lock(&rcu_registry_lock);
147 if (!QLIST_EMPTY(&registry)) {
148 /* In either case, the atomic_mb_set below blocks stores that free
149 * old RCU-protected pointers.
151 if (sizeof(rcu_gp_ctr) < 8) {
152 /* For architectures with 32-bit longs, a two-subphases algorithm
153 * ensures we do not encounter overflow bugs.
155 * Switch parity: 0 -> 1, 1 -> 0.
157 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
158 wait_for_readers();
159 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
160 } else {
161 /* Increment current grace period. */
162 atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
165 wait_for_readers();
168 qemu_mutex_unlock(&rcu_registry_lock);
169 qemu_mutex_unlock(&rcu_sync_lock);
173 #define RCU_CALL_MIN_SIZE 30
175 /* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
176 * from liburcu. Note that head is only used by the consumer.
178 static struct rcu_head dummy;
179 static struct rcu_head *head = &dummy, **tail = &dummy.next;
180 static int rcu_call_count;
181 static QemuEvent rcu_call_ready_event;
183 static void enqueue(struct rcu_head *node)
185 struct rcu_head **old_tail;
187 node->next = NULL;
188 old_tail = atomic_xchg(&tail, &node->next);
189 atomic_mb_set(old_tail, node);
192 static struct rcu_head *try_dequeue(void)
194 struct rcu_head *node, *next;
196 retry:
197 /* Test for an empty list, which we do not expect. Note that for
198 * the consumer head and tail are always consistent. The head
199 * is consistent because only the consumer reads/writes it.
200 * The tail, because it is the first step in the enqueuing.
201 * It is only the next pointers that might be inconsistent.
203 if (head == &dummy && atomic_mb_read(&tail) == &dummy.next) {
204 abort();
207 /* If the head node has NULL in its next pointer, the value is
208 * wrong and we need to wait until its enqueuer finishes the update.
210 node = head;
211 next = atomic_mb_read(&head->next);
212 if (!next) {
213 return NULL;
216 /* Since we are the sole consumer, and we excluded the empty case
217 * above, the queue will always have at least two nodes: the
218 * dummy node, and the one being removed. So we do not need to update
219 * the tail pointer.
221 head = next;
223 /* If we dequeued the dummy node, add it back at the end and retry. */
224 if (node == &dummy) {
225 enqueue(node);
226 goto retry;
229 return node;
232 static void *call_rcu_thread(void *opaque)
234 struct rcu_head *node;
236 rcu_register_thread();
238 for (;;) {
239 int tries = 0;
240 int n = atomic_read(&rcu_call_count);
242 /* Heuristically wait for a decent number of callbacks to pile up.
243 * Fetch rcu_call_count now, we only must process elements that were
244 * added before synchronize_rcu() starts.
246 while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
247 g_usleep(10000);
248 if (n == 0) {
249 qemu_event_reset(&rcu_call_ready_event);
250 n = atomic_read(&rcu_call_count);
251 if (n == 0) {
252 #if defined(CONFIG_MALLOC_TRIM)
253 malloc_trim(4 * 1024 * 1024);
254 #endif
255 qemu_event_wait(&rcu_call_ready_event);
258 n = atomic_read(&rcu_call_count);
261 atomic_sub(&rcu_call_count, n);
262 synchronize_rcu();
263 qemu_mutex_lock_iothread();
264 while (n > 0) {
265 node = try_dequeue();
266 while (!node) {
267 qemu_mutex_unlock_iothread();
268 qemu_event_reset(&rcu_call_ready_event);
269 node = try_dequeue();
270 if (!node) {
271 qemu_event_wait(&rcu_call_ready_event);
272 node = try_dequeue();
274 qemu_mutex_lock_iothread();
277 n--;
278 node->func(node);
280 qemu_mutex_unlock_iothread();
282 abort();
285 void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
287 node->func = func;
288 enqueue(node);
289 atomic_inc(&rcu_call_count);
290 qemu_event_set(&rcu_call_ready_event);
293 void rcu_register_thread(void)
295 assert(rcu_reader.ctr == 0);
296 qemu_mutex_lock(&rcu_registry_lock);
297 QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
298 qemu_mutex_unlock(&rcu_registry_lock);
301 void rcu_unregister_thread(void)
303 qemu_mutex_lock(&rcu_registry_lock);
304 QLIST_REMOVE(&rcu_reader, node);
305 qemu_mutex_unlock(&rcu_registry_lock);
308 static void rcu_init_complete(void)
310 QemuThread thread;
312 qemu_mutex_init(&rcu_registry_lock);
313 qemu_mutex_init(&rcu_sync_lock);
314 qemu_event_init(&rcu_gp_event, true);
316 qemu_event_init(&rcu_call_ready_event, false);
318 /* The caller is assumed to have iothread lock, so the call_rcu thread
319 * must have been quiescent even after forking, just recreate it.
321 qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
322 NULL, QEMU_THREAD_DETACHED);
324 rcu_register_thread();
327 static int atfork_depth = 1;
329 void rcu_enable_atfork(void)
331 atfork_depth++;
334 void rcu_disable_atfork(void)
336 atfork_depth--;
339 #ifdef CONFIG_POSIX
340 static void rcu_init_lock(void)
342 if (atfork_depth < 1) {
343 return;
346 qemu_mutex_lock(&rcu_sync_lock);
347 qemu_mutex_lock(&rcu_registry_lock);
350 static void rcu_init_unlock(void)
352 if (atfork_depth < 1) {
353 return;
356 qemu_mutex_unlock(&rcu_registry_lock);
357 qemu_mutex_unlock(&rcu_sync_lock);
360 static void rcu_init_child(void)
362 if (atfork_depth < 1) {
363 return;
366 memset(&registry, 0, sizeof(registry));
367 rcu_init_complete();
369 #endif
371 static void __attribute__((__constructor__)) rcu_init(void)
373 #ifdef CONFIG_POSIX
374 pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
375 #endif
376 rcu_init_complete();