qemu

rcu.c (13343B)

---

 /*
  * 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.
  */
 QEMU_DEFINE_CO_TLS(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(get_ptr_rcu_reader()->ctr == 0);
     qemu_mutex_lock(&rcu_registry_lock);
     QLIST_INSERT_HEAD(&registry, get_ptr_rcu_reader(), node);
     qemu_mutex_unlock(&rcu_registry_lock);
 }
 
 void rcu_unregister_thread(void)
 {
     qemu_mutex_lock(&rcu_registry_lock);
     QLIST_REMOVE(get_ptr_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(&get_ptr_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();
 }