--- /dev/null
+/*
+ * urcu-mb.c
+ *
+ * Userspace RCU library with explicit memory barriers
+ *
+ * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
+ * Copyright 2015 Red Hat, Inc.
+ *
+ * Ported to QEMU by Paolo Bonzini <pbonzini@redhat.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * IBM's contributions to this file may be relicensed under LGPLv2 or later.
+ */
+
+#include "qemu-common.h"
+#include <stdio.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <errno.h>
+#include "qemu/rcu.h"
+#include "qemu/atomic.h"
+#include "qemu/thread.h"
+#include "qemu/main-loop.h"
+
+/*
+ * Global grace period counter. Bit 0 is always one in rcu_gp_ctr.
+ * Bits 1 and above are defined in synchronize_rcu.
+ */
+#define RCU_GP_LOCKED (1UL << 0)
+#define RCU_GP_CTR (1UL << 1)
+
+unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
+
+QemuEvent rcu_gp_event;
+static QemuMutex rcu_gp_lock;
+
+/*
+ * Check whether a quiescent state was crossed between the beginning of
+ * update_counter_and_wait and now.
+ */
+static inline int rcu_gp_ongoing(unsigned long *ctr)
+{
+ unsigned long v;
+
+ v = atomic_read(ctr);
+ return v && (v != rcu_gp_ctr);
+}
+
+/* Written to only by each individual reader. Read by both the reader and the
+ * writers.
+ */
+__thread struct rcu_reader_data rcu_reader;
+
+/* Protected by rcu_gp_lock. */
+typedef QLIST_HEAD(, rcu_reader_data) ThreadList;
+static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
+
+/* Wait for previous parity/grace period to be empty of readers. */
+static void wait_for_readers(void)
+{
+ ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders);
+ struct rcu_reader_data *index, *tmp;
+
+ for (;;) {
+ /* We want to be notified of changes made to rcu_gp_ongoing
+ * while we walk the list.
+ */
+ qemu_event_reset(&rcu_gp_event);
+
+ /* Instead of using atomic_mb_set for index->waiting, and
+ * atomic_mb_read for index->ctr, memory barriers are placed
+ * manually since writes to different threads are independent.
+ * atomic_mb_set has a smp_wmb before...
+ */
+ smp_wmb();
+ QLIST_FOREACH(index, ®istry, node) {
+ atomic_set(&index->waiting, true);
+ }
+
+ /* ... and a smp_mb after. */
+ smp_mb();
+
+ QLIST_FOREACH_SAFE(index, ®istry, node, tmp) {
+ if (!rcu_gp_ongoing(&index->ctr)) {
+ QLIST_REMOVE(index, node);
+ QLIST_INSERT_HEAD(&qsreaders, index, node);
+
+ /* No need for mb_set here, worst of all we
+ * get some extra futex wakeups.
+ */
+ atomic_set(&index->waiting, false);
+ }
+ }
+
+ /* atomic_mb_read has smp_rmb after. */
+ smp_rmb();
+
+ if (QLIST_EMPTY(®istry)) {
+ break;
+ }
+
+ /* Wait for one thread to report a quiescent state and
+ * try again.
+ */
+ qemu_event_wait(&rcu_gp_event);
+ }
+
+ /* put back the reader list in the registry */
+ QLIST_SWAP(®istry, &qsreaders, node);
+}
+
+void synchronize_rcu(void)
+{
+ qemu_mutex_lock(&rcu_gp_lock);
+
+ if (!QLIST_EMPTY(®istry)) {
+ /* In either case, the atomic_mb_set below blocks stores that free
+ * old RCU-protected pointers.
+ */
+ if (sizeof(rcu_gp_ctr) < 8) {
+ /* For architectures with 32-bit longs, a two-subphases algorithm
+ * ensures we do not encounter overflow bugs.
+ *
+ * Switch parity: 0 -> 1, 1 -> 0.
+ */
+ atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+ wait_for_readers();
+ atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+ } else {
+ /* Increment current grace period. */
+ atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+ }
+
+ wait_for_readers();
+ }
+
+ qemu_mutex_unlock(&rcu_gp_lock);
+}
+
+
+#define RCU_CALL_MIN_SIZE 30
+
+/* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
+ * from liburcu. Note that head is only used by the consumer.
+ */
+static struct rcu_head dummy;
+static struct rcu_head *head = &dummy, **tail = &dummy.next;
+static int rcu_call_count;
+static QemuEvent rcu_call_ready_event;
+
+static void enqueue(struct rcu_head *node)
+{
+ struct rcu_head **old_tail;
+
+ node->next = NULL;
+ old_tail = atomic_xchg(&tail, &node->next);
+ atomic_mb_set(old_tail, node);
+}
+
+static struct rcu_head *try_dequeue(void)
+{
+ struct rcu_head *node, *next;
+
+retry:
+ /* Test for an empty list, which we do not expect. Note that for
+ * the consumer head and tail are always consistent. The head
+ * is consistent because only the consumer reads/writes it.
+ * The tail, because it is the first step in the enqueuing.
+ * It is only the next pointers that might be inconsistent.
+ */
+ if (head == &dummy && atomic_mb_read(&tail) == &dummy.next) {
+ abort();
+ }
+
+ /* If the head node has NULL in its next pointer, the value is
+ * wrong and we need to wait until its enqueuer finishes the update.
+ */
+ node = head;
+ next = atomic_mb_read(&head->next);
+ if (!next) {
+ return NULL;
+ }
+
+ /* Since we are the sole consumer, and we excluded the empty case
+ * above, the queue will always have at least two nodes: the
+ * dummy node, and the one being removed. So we do not need to update
+ * the tail pointer.
+ */
+ head = next;
+
+ /* If we dequeued the dummy node, add it back at the end and retry. */
+ if (node == &dummy) {
+ enqueue(node);
+ goto retry;
+ }
+
+ return node;
+}
+
+static void *call_rcu_thread(void *opaque)
+{
+ struct rcu_head *node;
+
+ rcu_register_thread();
+
+ for (;;) {
+ int tries = 0;
+ int n = atomic_read(&rcu_call_count);
+
+ /* Heuristically wait for a decent number of callbacks to pile up.
+ * Fetch rcu_call_count now, we only must process elements that were
+ * added before synchronize_rcu() starts.
+ */
+ while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
+ g_usleep(10000);
+ if (n == 0) {
+ qemu_event_reset(&rcu_call_ready_event);
+ n = atomic_read(&rcu_call_count);
+ if (n == 0) {
+ qemu_event_wait(&rcu_call_ready_event);
+ }
+ }
+ n = atomic_read(&rcu_call_count);
+ }
+
+ atomic_sub(&rcu_call_count, n);
+ synchronize_rcu();
+ qemu_mutex_lock_iothread();
+ while (n > 0) {
+ node = try_dequeue();
+ while (!node) {
+ qemu_mutex_unlock_iothread();
+ qemu_event_reset(&rcu_call_ready_event);
+ node = try_dequeue();
+ if (!node) {
+ qemu_event_wait(&rcu_call_ready_event);
+ node = try_dequeue();
+ }
+ qemu_mutex_lock_iothread();
+ }
+
+ n--;
+ node->func(node);
+ }
+ qemu_mutex_unlock_iothread();
+ }
+ abort();
+}
+
+void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
+{
+ node->func = func;
+ enqueue(node);
+ atomic_inc(&rcu_call_count);
+ qemu_event_set(&rcu_call_ready_event);
+}
+
+void rcu_register_thread(void)
+{
+ assert(rcu_reader.ctr == 0);
+ qemu_mutex_lock(&rcu_gp_lock);
+ QLIST_INSERT_HEAD(®istry, &rcu_reader, node);
+ qemu_mutex_unlock(&rcu_gp_lock);
+}
+
+void rcu_unregister_thread(void)
+{
+ qemu_mutex_lock(&rcu_gp_lock);
+ QLIST_REMOVE(&rcu_reader, node);
+ qemu_mutex_unlock(&rcu_gp_lock);
+}
+
+static void rcu_init_complete(void)
+{
+ QemuThread thread;
+
+ qemu_mutex_init(&rcu_gp_lock);
+ qemu_event_init(&rcu_gp_event, true);
+
+ qemu_event_init(&rcu_call_ready_event, false);
+
+ /* The caller is assumed to have iothread lock, so the call_rcu thread
+ * must have been quiescent even after forking, just recreate it.
+ */
+ qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
+ NULL, QEMU_THREAD_DETACHED);
+
+ rcu_register_thread();
+}
+
+#ifdef CONFIG_POSIX
+static void rcu_init_lock(void)
+{
+ qemu_mutex_lock(&rcu_gp_lock);
+}
+
+static void rcu_init_unlock(void)
+{
+ qemu_mutex_unlock(&rcu_gp_lock);
+}
+#endif
+
+void rcu_after_fork(void)
+{
+ memset(®istry, 0, sizeof(registry));
+ rcu_init_complete();
+}
+
+static void __attribute__((__constructor__)) rcu_init(void)
+{
+#ifdef CONFIG_POSIX
+ pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_unlock);
+#endif
+ rcu_init_complete();
+}
Code Review / kvmfornfv.git / blobdiff