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-common.h"
  30 #include <stdio.h>
  31 #include <assert.h>
  32 #include <stdlib.h>
  33 #include <stdint.h>
  34 #include <errno.h>
  35 #include "qemu/rcu.h"
  36 #include "qemu/atomic.h"
  37 #include "qemu/thread.h"
  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 QemuMutex rcu_gp_lock;
  50
  51 /*
  52  * Check whether a quiescent state was crossed between the beginning of
  53  * update_counter_and_wait and now.
  54  */
  55 static 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_gp_lock.  */
  69 typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
  70 static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
  71
  72 /* Wait for previous parity/grace period to be empty of readers.  */
  73 static 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          * atomic_mb_set has a smp_wmb before...
  88          */
  89         smp_wmb();
  90         QLIST_FOREACH(index, &registry, node) {
  91             atomic_set(&index->waiting, true);
  92         }
  93
  94         /* ... and a smp_mb after.  */
  95         smp_mb();
  96
  97         QLIST_FOREACH_SAFE(index, &registry, node, tmp) {
  98             if (!rcu_gp_ongoing(&index->ctr)) {
  99                 QLIST_REMOVE(index, node);
 100                 QLIST_INSERT_HEAD(&qsreaders, index, node);
 101
 102                 /* No need for mb_set here, worst of all we
 103                  * get some extra futex wakeups.
 104                  */
 105                 atomic_set(&index->waiting, false);
 106             }
 107         }
 108
 109         /* atomic_mb_read has smp_rmb after.  */
 110         smp_rmb();
 111
 112         if (QLIST_EMPTY(&registry)) {
 113             break;
 114         }
 115
 116         /* Wait for one thread to report a quiescent state and
 117          * try again.
 118          */
 119         qemu_event_wait(&rcu_gp_event);
 120     }
 121
 122     /* put back the reader list in the registry */
 123     QLIST_SWAP(&registry, &qsreaders, node);
 124 }
 125
 126 void synchronize_rcu(void)
 127 {
 128     qemu_mutex_lock(&rcu_gp_lock);
 129
 130     if (!QLIST_EMPTY(&registry)) {
 131         /* In either case, the atomic_mb_set below blocks stores that free
 132          * old RCU-protected pointers.
 133          */
 134         if (sizeof(rcu_gp_ctr) < 8) {
 135             /* For architectures with 32-bit longs, a two-subphases algorithm
 136              * ensures we do not encounter overflow bugs.
 137              *
 138              * Switch parity: 0 -> 1, 1 -> 0.
 139              */
 140             atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 141             wait_for_readers();
 142             atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 143         } else {
 144             /* Increment current grace period.  */
 145             atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 146         }
 147
 148         wait_for_readers();
 149     }
 150
 151     qemu_mutex_unlock(&rcu_gp_lock);
 152 }
 153
 154
 155 #define RCU_CALL_MIN_SIZE        30
 156
 157 /* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
 158  * from liburcu.  Note that head is only used by the consumer.
 159  */
 160 static struct rcu_head dummy;
 161 static struct rcu_head *head = &dummy, **tail = &dummy.next;
 162 static int rcu_call_count;
 163 static QemuEvent rcu_call_ready_event;
 164
 165 static void enqueue(struct rcu_head *node)
 166 {
 167     struct rcu_head **old_tail;
 168
 169     node->next = NULL;
 170     old_tail = atomic_xchg(&tail, &node->next);
 171     atomic_mb_set(old_tail, node);
 172 }
 173
 174 static struct rcu_head *try_dequeue(void)
 175 {
 176     struct rcu_head *node, *next;
 177
 178 retry:
 179     /* Test for an empty list, which we do not expect.  Note that for
 180      * the consumer head and tail are always consistent.  The head
 181      * is consistent because only the consumer reads/writes it.
 182      * The tail, because it is the first step in the enqueuing.
 183      * It is only the next pointers that might be inconsistent.
 184      */
 185     if (head == &dummy && atomic_mb_read(&tail) == &dummy.next) {
 186         abort();
 187     }
 188
 189     /* If the head node has NULL in its next pointer, the value is
 190      * wrong and we need to wait until its enqueuer finishes the update.
 191      */
 192     node = head;
 193     next = atomic_mb_read(&head->next);
 194     if (!next) {
 195         return NULL;
 196     }
 197
 198     /* Since we are the sole consumer, and we excluded the empty case
 199      * above, the queue will always have at least two nodes: the
 200      * dummy node, and the one being removed.  So we do not need to update
 201      * the tail pointer.
 202      */
 203     head = next;
 204
 205     /* If we dequeued the dummy node, add it back at the end and retry.  */
 206     if (node == &dummy) {
 207         enqueue(node);
 208         goto retry;
 209     }
 210
 211     return node;
 212 }
 213
 214 static void *call_rcu_thread(void *opaque)
 215 {
 216     struct rcu_head *node;
 217
 218     for (;;) {
 219         int tries = 0;
 220         int n = atomic_read(&rcu_call_count);
 221
 222         /* Heuristically wait for a decent number of callbacks to pile up.
 223          * Fetch rcu_call_count now, we only must process elements that were
 224          * added before synchronize_rcu() starts.
 225          */
 226         while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
 227             g_usleep(10000);
 228             if (n == 0) {
 229                 qemu_event_reset(&rcu_call_ready_event);
 230                 n = atomic_read(&rcu_call_count);
 231                 if (n == 0) {
 232                     qemu_event_wait(&rcu_call_ready_event);
 233                 }
 234             }
 235             n = atomic_read(&rcu_call_count);
 236         }
 237
 238         atomic_sub(&rcu_call_count, n);
 239         synchronize_rcu();
 240         while (n > 0) {
 241             node = try_dequeue();
 242             while (!node) {
 243                 qemu_event_reset(&rcu_call_ready_event);
 244                 node = try_dequeue();
 245                 if (!node) {
 246                     qemu_event_wait(&rcu_call_ready_event);
 247                     node = try_dequeue();
 248                 }
 249             }
 250
 251             n--;
 252             node->func(node);
 253         }
 254     }
 255     abort();
 256 }
 257
 258 void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
 259 {
 260     node->func = func;
 261     enqueue(node);
 262     atomic_inc(&rcu_call_count);
 263     qemu_event_set(&rcu_call_ready_event);
 264 }
 265
 266 void rcu_register_thread(void)
 267 {
 268     assert(rcu_reader.ctr == 0);
 269     qemu_mutex_lock(&rcu_gp_lock);
 270     QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
 271     qemu_mutex_unlock(&rcu_gp_lock);
 272 }
 273
 274 void rcu_unregister_thread(void)
 275 {
 276     qemu_mutex_lock(&rcu_gp_lock);
 277     QLIST_REMOVE(&rcu_reader, node);
 278     qemu_mutex_unlock(&rcu_gp_lock);
 279 }
 280
 281 static void __attribute__((__constructor__)) rcu_init(void)
 282 {
 283     QemuThread thread;
 284
 285     qemu_mutex_init(&rcu_gp_lock);
 286     qemu_event_init(&rcu_gp_event, true);
 287
 288     qemu_event_init(&rcu_call_ready_event, false);
 289     qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
 290                        NULL, QEMU_THREAD_DETACHED);
 291
 292     rcu_register_thread();
 293 }