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Diffstat (limited to 'util/rcu.c')
| -rw-r--r-- | util/rcu.c | 172 |
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, ®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); +} + +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 __attribute__((__constructor__)) rcu_init(void) +{ + qemu_mutex_init(&rcu_gp_lock); + qemu_event_init(&rcu_gp_event, true); + rcu_register_thread(); +} |