1 files changed, 172 insertions, 0 deletions
diff --git a/util/rcu.c b/util/rcu.c
new file mode 100644
index 0000000000..1f737d5447
--- /dev/null
+++ b/util/rcu.c
@@ -0,0 +1,172 @@
+/*
+ * 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 <stdio.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <errno.h>
+#include "qemu/rcu.h"
+#include "qemu/atomic.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, &registry, node) {
+            atomic_set(&index->waiting, true);
+        }
+
+        /* ... and a smp_mb after.  */
+        smp_mb();
+
+        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.
+                 */
+                atomic_set(&index->waiting, false);
+            }
+        }
+
+        /* atomic_mb_read has smp_rmb after.  */
+        smp_rmb();
+
+        if (QLIST_EMPTY(&registry)) {
+            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(&registry, &qsreaders, node);
+}
+
+void synchronize_rcu(void)
+{
+    qemu_mutex_lock(&rcu_gp_lock);
+
+    if (!QLIST_EMPTY(&registry)) {
+        /* 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);
+}
+
+void rcu_register_thread(void)
+{
+    assert(rcu_reader.ctr == 0);
+    qemu_mutex_lock(&rcu_gp_lock);
+    QLIST_INSERT_HEAD(&registry, &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 __attribute__((__constructor__)) rcu_init(void)
+{
+    qemu_mutex_init(&rcu_gp_lock);
+    qemu_event_init(&rcu_gp_event, true);
+    rcu_register_thread();
+}