| /* |
| * 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; |
| |
| 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); |
| } |
| |
| static void rcu_init_child(void) |
| { |
| qemu_mutex_unlock(&rcu_gp_lock); |
| memset(®istry, 0, sizeof(registry)); |
| rcu_init_complete(); |
| } |
| #endif |
| |
| static void __attribute__((__constructor__)) rcu_init(void) |
| { |
| #ifdef CONFIG_POSIX |
| pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child); |
| #endif |
| rcu_init_complete(); |
| } |