1 files changed, 455 insertions, 0 deletions
diff --git a/util/rcu.c b/util/rcu.c
new file mode 100644
index 000000000..c91da9f13
--- /dev/null
+++ b/util/rcu.c
@@ -0,0 +1,455 @@
+/*
+ * 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/osdep.h"
+#include "qemu/rcu.h"
+#include "qemu/atomic.h"
+#include "qemu/thread.h"
+#include "qemu/main-loop.h"
+#include "qemu/lockable.h"
+#if defined(CONFIG_MALLOC_TRIM)
+#include <malloc.h>
+#endif
+
+/*
+ * 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 int in_drain_call_rcu;
+static QemuMutex rcu_registry_lock;
+static QemuMutex rcu_sync_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 = qatomic_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_registry_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 qatomic_mb_set for index->waiting, and
+         * qatomic_mb_read for index->ctr, memory barriers are placed
+         * manually since writes to different threads are independent.
+         * qemu_event_reset has acquire semantics, so no memory barrier
+         * is needed here.
+         */
+        QLIST_FOREACH(index, &registry, node) {
+            qatomic_set(&index->waiting, true);
+        }
+
+        /* Here, order the stores to index->waiting before the loads of
+         * index->ctr.  Pairs with smp_mb_placeholder() in rcu_read_unlock(),
+         * ensuring that the loads of index->ctr are sequentially consistent.
+         */
+        smp_mb_global();
+
+        QLIST_FOREACH_SAFE(index, &registry, 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.
+                 */
+                qatomic_set(&index->waiting, false);
+            } else if (qatomic_read(&in_drain_call_rcu)) {
+                notifier_list_notify(&index->force_rcu, NULL);
+            }
+        }
+
+        if (QLIST_EMPTY(&registry)) {
+            break;
+        }
+
+        /* Wait for one thread to report a quiescent state and try again.
+         * Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
+         * wait too much time.
+         *
+         * rcu_register_thread() may add nodes to &registry; it will not
+         * wake up synchronize_rcu, but that is okay because at least another
+         * thread must exit its RCU read-side critical section before
+         * synchronize_rcu is done.  The next iteration of the loop will
+         * move the new thread's rcu_reader from &registry to &qsreaders,
+         * because rcu_gp_ongoing() will return false.
+         *
+         * rcu_unregister_thread() may remove nodes from &qsreaders instead
+         * of &registry if it runs during qemu_event_wait.  That's okay;
+         * the node then will not be added back to &registry by QLIST_SWAP
+         * below.  The invariant is that the node is part of one list when
+         * rcu_registry_lock is released.
+         */
+        qemu_mutex_unlock(&rcu_registry_lock);
+        qemu_event_wait(&rcu_gp_event);
+        qemu_mutex_lock(&rcu_registry_lock);
+    }
+
+    /* put back the reader list in the registry */
+    QLIST_SWAP(&registry, &qsreaders, node);
+}
+
+void synchronize_rcu(void)
+{
+    QEMU_LOCK_GUARD(&rcu_sync_lock);
+
+    /* Write RCU-protected pointers before reading p_rcu_reader->ctr.
+     * Pairs with smp_mb_placeholder() in rcu_read_lock().
+     */
+    smp_mb_global();
+
+    QEMU_LOCK_GUARD(&rcu_registry_lock);
+    if (!QLIST_EMPTY(&registry)) {
+        /* In either case, the qatomic_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.
+             */
+            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+            wait_for_readers();
+            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+        } else {
+            /* Increment current grace period.  */
+            qatomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+        }
+
+        wait_for_readers();
+    }
+}
+
+
+#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 = qatomic_xchg(&tail, &node->next);
+    qatomic_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 && qatomic_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 = qatomic_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 = qatomic_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 = qatomic_read(&rcu_call_count);
+                if (n == 0) {
+#if defined(CONFIG_MALLOC_TRIM)
+                    malloc_trim(4 * 1024 * 1024);
+#endif
+                    qemu_event_wait(&rcu_call_ready_event);
+                }
+            }
+            n = qatomic_read(&rcu_call_count);
+        }
+
+        qatomic_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);
+    qatomic_inc(&rcu_call_count);
+    qemu_event_set(&rcu_call_ready_event);
+}
+
+
+struct rcu_drain {
+    struct rcu_head rcu;
+    QemuEvent drain_complete_event;
+};
+
+static void drain_rcu_callback(struct rcu_head *node)
+{
+    struct rcu_drain *event = (struct rcu_drain *)node;
+    qemu_event_set(&event->drain_complete_event);
+}
+
+/*
+ * This function ensures that all pending RCU callbacks
+ * on the current thread are done executing
+
+ * drops big qemu lock during the wait to allow RCU thread
+ * to process the callbacks
+ *
+ */
+
+void drain_call_rcu(void)
+{
+    struct rcu_drain rcu_drain;
+    bool locked = qemu_mutex_iothread_locked();
+
+    memset(&rcu_drain, 0, sizeof(struct rcu_drain));
+    qemu_event_init(&rcu_drain.drain_complete_event, false);
+
+    if (locked) {
+        qemu_mutex_unlock_iothread();
+    }
+
+
+    /*
+     * RCU callbacks are invoked in the same order as in which they
+     * are registered, thus we can be sure that when 'drain_rcu_callback'
+     * is called, all RCU callbacks that were registered on this thread
+     * prior to calling this function are completed.
+     *
+     * Note that since we have only one global queue of the RCU callbacks,
+     * we also end up waiting for most of RCU callbacks that were registered
+     * on the other threads, but this is a side effect that shoudn't be
+     * assumed.
+     */
+
+    qatomic_inc(&in_drain_call_rcu);
+    call_rcu1(&rcu_drain.rcu, drain_rcu_callback);
+    qemu_event_wait(&rcu_drain.drain_complete_event);
+    qatomic_dec(&in_drain_call_rcu);
+
+    if (locked) {
+        qemu_mutex_lock_iothread();
+    }
+
+}
+
+void rcu_register_thread(void)
+{
+    assert(rcu_reader.ctr == 0);
+    qemu_mutex_lock(&rcu_registry_lock);
+    QLIST_INSERT_HEAD(&registry, &rcu_reader, node);
+    qemu_mutex_unlock(&rcu_registry_lock);
+}
+
+void rcu_unregister_thread(void)
+{
+    qemu_mutex_lock(&rcu_registry_lock);
+    QLIST_REMOVE(&rcu_reader, node);
+    qemu_mutex_unlock(&rcu_registry_lock);
+}
+
+void rcu_add_force_rcu_notifier(Notifier *n)
+{
+    qemu_mutex_lock(&rcu_registry_lock);
+    notifier_list_add(&rcu_reader.force_rcu, n);
+    qemu_mutex_unlock(&rcu_registry_lock);
+}
+
+void rcu_remove_force_rcu_notifier(Notifier *n)
+{
+    qemu_mutex_lock(&rcu_registry_lock);
+    notifier_remove(n);
+    qemu_mutex_unlock(&rcu_registry_lock);
+}
+
+static void rcu_init_complete(void)
+{
+    QemuThread thread;
+
+    qemu_mutex_init(&rcu_registry_lock);
+    qemu_mutex_init(&rcu_sync_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();
+}
+
+static int atfork_depth = 1;
+
+void rcu_enable_atfork(void)
+{
+    atfork_depth++;
+}
+
+void rcu_disable_atfork(void)
+{
+    atfork_depth--;
+}
+
+#ifdef CONFIG_POSIX
+static void rcu_init_lock(void)
+{
+    if (atfork_depth < 1) {
+        return;
+    }
+
+    qemu_mutex_lock(&rcu_sync_lock);
+    qemu_mutex_lock(&rcu_registry_lock);
+}
+
+static void rcu_init_unlock(void)
+{
+    if (atfork_depth < 1) {
+        return;
+    }
+
+    qemu_mutex_unlock(&rcu_registry_lock);
+    qemu_mutex_unlock(&rcu_sync_lock);
+}
+
+static void rcu_init_child(void)
+{
+    if (atfork_depth < 1) {
+        return;
+    }
+
+    memset(&registry, 0, sizeof(registry));
+    rcu_init_complete();
+}
+#endif
+
+static void __attribute__((__constructor__)) rcu_init(void)
+{
+    smp_mb_global_init();
+#ifdef CONFIG_POSIX
+    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
+#endif
+    rcu_init_complete();
+}