qemu/util/rcu.c
Paolo Bonzini 8f593ba9c5 call_rcu: stop using mb_set/mb_read
Use a store-release when enqueuing a new call_rcu, and a load-acquire
when dequeuing; and read the tail after checking that node->next is
consistent, which is the standard message passing pattern and it is
clearer than mb_read/mb_set.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2023-05-08 11:10:49 +02:00

473 lines
13 KiB
C

/*
* 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);
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.
*
* If this is the last iteration, this barrier also prevents
* frees from seeping upwards, and orders the two wait phases
* on architectures with 32-bit longs; see synchronize_rcu().
*/
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 memory barriers 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().
*
* Also orders write to RCU-protected pointers before
* write to rcu_gp_ctr.
*/
smp_mb_global();
QEMU_LOCK_GUARD(&rcu_registry_lock);
if (!QLIST_EMPTY(&registry)) {
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_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
wait_for_readers();
qatomic_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
} else {
/* Increment current grace period. */
qatomic_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;
/*
* Make this node the tail of the list. The node will be
* used by further enqueue operations, but it will not
* be dequeued yet...
*/
old_tail = qatomic_xchg(&tail, &node->next);
/*
* ... until it is pointed to from another item in the list.
* In the meantime, try_dequeue() will find a NULL next pointer
* and loop.
*
* Synchronizes with qatomic_load_acquire() in try_dequeue().
*/
qatomic_store_release(old_tail, node);
}
static struct rcu_head *try_dequeue(void)
{
struct rcu_head *node, *next;
retry:
/* Head is only written by this thread, so no need for barriers. */
node = head;
/*
* 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.
*/
next = qatomic_load_acquire(&node->next);
if (!next) {
return NULL;
}
/*
* 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_read(&tail) == &dummy.next) {
abort();
}
/*
* 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();
}