/* * 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-common.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_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 = 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_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 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. * qemu_event_reset has acquire semantics, so no memory barrier * is needed here. */ QLIST_FOREACH(index, ®istry, node) { atomic_set(&index->waiting, true); } /* Here, order the stores to index->waiting before the * loads of index->ctr. */ 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); } } if (QLIST_EMPTY(®istry)) { 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 ®istry; 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 ®istry to &qsreaders, * because rcu_gp_ongoing() will return false. * * rcu_unregister_thread() may remove nodes from &qsreaders instead * of ®istry if it runs during qemu_event_wait. That's okay; * the node then will not be added back to ®istry 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(®istry, &qsreaders, node); } void synchronize_rcu(void) { qemu_mutex_lock(&rcu_sync_lock); qemu_mutex_lock(&rcu_registry_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_registry_lock); qemu_mutex_unlock(&rcu_sync_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_registry_lock); QLIST_INSERT_HEAD(®istry, &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); } 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(); } #ifdef CONFIG_POSIX static void rcu_init_lock(void) { qemu_mutex_lock(&rcu_sync_lock); qemu_mutex_lock(&rcu_registry_lock); } static void rcu_init_unlock(void) { qemu_mutex_unlock(&rcu_registry_lock); qemu_mutex_unlock(&rcu_sync_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(); }