/*
* coroutine queues and locks
*
* Copyright (c) 2011 Kevin Wolf <kwolf@redhat.com>
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*
* The lock-free mutex implementation is based on OSv
* (core/lfmutex.cc, include/lockfree/mutex.hh).
* Copyright (C) 2013 Cloudius Systems, Ltd.
*/
#include "qemu/osdep.h"
#include "qemu/coroutine.h"
#include "qemu/coroutine_int.h"
#include "qemu/processor.h"
#include "qemu/queue.h"
#include "block/aio.h"
#include "trace.h"
void qemu_co_queue_init(CoQueue *queue)
{
QSIMPLEQ_INIT(&queue->entries);
}
void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock)
{
Coroutine *self = qemu_coroutine_self();
QSIMPLEQ_INSERT_TAIL(&queue->entries, self, co_queue_next);
if (lock) {
qemu_lockable_unlock(lock);
}
/* There is no race condition here. Other threads will call
* aio_co_schedule on our AioContext, which can reenter this
* coroutine but only after this yield and after the main loop
* has gone through the next iteration.
*/
qemu_coroutine_yield();
assert(qemu_in_coroutine());
/* TODO: OSv implements wait morphing here, where the wakeup
* primitive automatically places the woken coroutine on the
* mutex's queue. This avoids the thundering herd effect.
* This could be implemented for CoMutexes, but not really for
* other cases of QemuLockable.
*/
if (lock) {
qemu_lockable_lock(lock);
}
}
static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
{
Coroutine *next;
if (QSIMPLEQ_EMPTY(&queue->entries)) {
return false;
}
while ((next = QSIMPLEQ_FIRST(&queue->entries)) != NULL) {
QSIMPLEQ_REMOVE_HEAD(&queue->entries, co_queue_next);
aio_co_wake(next);
if (single) {
break;
}
}
return true;
}
bool qemu_co_queue_next(CoQueue *queue)
{
return qemu_co_queue_do_restart(queue, true);
}
void qemu_co_queue_restart_all(CoQueue *queue)
{
qemu_co_queue_do_restart(queue, false);
}
bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock)
{
Coroutine *next;
next = QSIMPLEQ_FIRST(&queue->entries);
if (!next) {
return false;
}
QSIMPLEQ_REMOVE_HEAD(&queue->entries, co_queue_next);
if (lock) {
qemu_lockable_unlock(lock);
}
aio_co_wake(next);
if (lock) {
qemu_lockable_lock(lock);
}
return true;
}
bool qemu_co_queue_empty(CoQueue *queue)
{
return QSIMPLEQ_FIRST(&queue->entries) == NULL;
}
/* The wait records are handled with a multiple-producer, single-consumer
* lock-free queue. There cannot be two concurrent pop_waiter() calls
* because pop_waiter() can only be called while mutex->handoff is zero.
* This can happen in three cases:
* - in qemu_co_mutex_unlock, before the hand-off protocol has started.
* In this case, qemu_co_mutex_lock will see mutex->handoff == 0 and
* not take part in the handoff.
* - in qemu_co_mutex_lock, if it steals the hand-off responsibility from
* qemu_co_mutex_unlock. In this case, qemu_co_mutex_unlock will fail
* the cmpxchg (it will see either 0 or the next sequence value) and
* exit. The next hand-off cannot begin until qemu_co_mutex_lock has
* woken up someone.
* - in qemu_co_mutex_unlock, if it takes the hand-off token itself.
* In this case another iteration starts with mutex->handoff == 0;
* a concurrent qemu_co_mutex_lock will fail the cmpxchg, and
* qemu_co_mutex_unlock will go back to case (1).
*
* The following functions manage this queue.
*/
typedef struct CoWaitRecord {
Coroutine *co;
QSLIST_ENTRY(CoWaitRecord) next;
} CoWaitRecord;
static void push_waiter(CoMutex *mutex, CoWaitRecord *w)
{
w->co = qemu_coroutine_self();
QSLIST_INSERT_HEAD_ATOMIC(&mutex->from_push, w, next);
}
static void move_waiters(CoMutex *mutex)
{
QSLIST_HEAD(, CoWaitRecord) reversed;
QSLIST_MOVE_ATOMIC(&reversed, &mutex->from_push);
while (!QSLIST_EMPTY(&reversed)) {
CoWaitRecord *w = QSLIST_FIRST(&reversed);
QSLIST_REMOVE_HEAD(&reversed, next);
QSLIST_INSERT_HEAD(&mutex->to_pop, w, next);
}
}
static CoWaitRecord *pop_waiter(CoMutex *mutex)
{
CoWaitRecord *w;
if (QSLIST_EMPTY(&mutex->to_pop)) {
move_waiters(mutex);
if (QSLIST_EMPTY(&mutex->to_pop)) {
return NULL;
}
}
w = QSLIST_FIRST(&mutex->to_pop);
QSLIST_REMOVE_HEAD(&mutex->to_pop, next);
return w;
}
static bool has_waiters(CoMutex *mutex)
{
return QSLIST_EMPTY(&mutex->to_pop) || QSLIST_EMPTY(&mutex->from_push);
}
void qemu_co_mutex_init(CoMutex *mutex)
{
memset(mutex, 0, sizeof(*mutex));
}
static void coroutine_fn qemu_co_mutex_wake(CoMutex *mutex, Coroutine *co)
{
/* Read co before co->ctx; pairs with smp_wmb() in
* qemu_coroutine_enter().
*/
smp_read_barrier_depends();
mutex->ctx = co->ctx;
aio_co_wake(co);
}
static void coroutine_fn qemu_co_mutex_lock_slowpath(AioContext *ctx,
CoMutex *mutex)
{
Coroutine *self = qemu_coroutine_self();
CoWaitRecord w;
unsigned old_handoff;
trace_qemu_co_mutex_lock_entry(mutex, self);
w.co = self;
push_waiter(mutex, &w);
/* This is the "Responsibility Hand-Off" protocol; a lock() picks from
* a concurrent unlock() the responsibility of waking somebody up.
*/
old_handoff = qatomic_mb_read(&mutex->handoff);
if (old_handoff &&
has_waiters(mutex) &&
qatomic_cmpxchg(&mutex->handoff, old_handoff, 0) == old_handoff) {
/* There can be no concurrent pops, because there can be only
* one active handoff at a time.
*/
CoWaitRecord *to_wake = pop_waiter(mutex);
Coroutine *co = to_wake->co;
if (co == self) {
/* We got the lock ourselves! */
assert(to_wake == &w);
mutex->ctx = ctx;
return;
}
qemu_co_mutex_wake(mutex, co);
}
qemu_coroutine_yield();
trace_qemu_co_mutex_lock_return(mutex, self);
}
void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex)
{
AioContext *ctx = qemu_get_current_aio_context();
Coroutine *self = qemu_coroutine_self();
int waiters, i;
/* Running a very small critical section on pthread_mutex_t and CoMutex
* shows that pthread_mutex_t is much faster because it doesn't actually
* go to sleep. What happens is that the critical section is shorter
* than the latency of entering the kernel and thus FUTEX_WAIT always
* fails. With CoMutex there is no such latency but you still want to
* avoid wait and wakeup. So introduce it artificially.
*/
i = 0;
retry_fast_path:
waiters = qatomic_cmpxchg(&mutex->locked, 0, 1);
if (waiters != 0) {
while (waiters == 1 && ++i < 1000) {
if (qatomic_read(&mutex->ctx) == ctx) {
break;
}
if (qatomic_read(&mutex->locked) == 0) {
goto retry_fast_path;
}
cpu_relax();
}
waiters = qatomic_fetch_inc(&mutex->locked);
}
if (waiters == 0) {
/* Uncontended. */
trace_qemu_co_mutex_lock_uncontended(mutex, self);
mutex->ctx = ctx;
} else {
qemu_co_mutex_lock_slowpath(ctx, mutex);
}
mutex->holder = self;
self->locks_held++;
}
void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex)
{
Coroutine *self = qemu_coroutine_self();
trace_qemu_co_mutex_unlock_entry(mutex, self);
assert(mutex->locked);
assert(mutex->holder == self);
assert(qemu_in_coroutine());
mutex->ctx = NULL;
mutex->holder = NULL;
self->locks_held--;
if (qatomic_fetch_dec(&mutex->locked) == 1) {
/* No waiting qemu_co_mutex_lock(). Pfew, that was easy! */
return;
}
for (;;) {
CoWaitRecord *to_wake = pop_waiter(mutex);
unsigned our_handoff;
if (to_wake) {
qemu_co_mutex_wake(mutex, to_wake->co);
break;
}
/* Some concurrent lock() is in progress (we know this because
* mutex->locked was >1) but it hasn't yet put itself on the wait
* queue. Pick a sequence number for the handoff protocol (not 0).
*/
if (++mutex->sequence == 0) {
mutex->sequence = 1;
}
our_handoff = mutex->sequence;
qatomic_mb_set(&mutex->handoff, our_handoff);
if (!has_waiters(mutex)) {
/* The concurrent lock has not added itself yet, so it
* will be able to pick our handoff.
*/
break;
}
/* Try to do the handoff protocol ourselves; if somebody else has
* already taken it, however, we're done and they're responsible.
*/
if (qatomic_cmpxchg(&mutex->handoff, our_handoff, 0) != our_handoff) {
break;
}
}
trace_qemu_co_mutex_unlock_return(mutex, self);
}
void qemu_co_rwlock_init(CoRwlock *lock)
{
memset(lock, 0, sizeof(*lock));
qemu_co_queue_init(&lock->queue);
qemu_co_mutex_init(&lock->mutex);
}
void qemu_co_rwlock_rdlock(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
qemu_co_mutex_lock(&lock->mutex);
/* For fairness, wait if a writer is in line. */
while (lock->pending_writer) {
qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
lock->reader++;
qemu_co_mutex_unlock(&lock->mutex);
/* The rest of the read-side critical section is run without the mutex. */
self->locks_held++;
}
void qemu_co_rwlock_unlock(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
assert(qemu_in_coroutine());
if (!lock->reader) {
/* The critical section started in qemu_co_rwlock_wrlock. */
qemu_co_queue_restart_all(&lock->queue);
} else {
self->locks_held--;
qemu_co_mutex_lock(&lock->mutex);
lock->reader--;
assert(lock->reader >= 0);
/* Wakeup only one waiting writer */
if (!lock->reader) {
qemu_co_queue_next(&lock->queue);
}
}
qemu_co_mutex_unlock(&lock->mutex);
}
void qemu_co_rwlock_downgrade(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
/* lock->mutex critical section started in qemu_co_rwlock_wrlock or
* qemu_co_rwlock_upgrade.
*/
assert(lock->reader == 0);
lock->reader++;
qemu_co_mutex_unlock(&lock->mutex);
/* The rest of the read-side critical section is run without the mutex. */
self->locks_held++;
}
void qemu_co_rwlock_wrlock(CoRwlock *lock)
{
qemu_co_mutex_lock(&lock->mutex);
lock->pending_writer++;
while (lock->reader) {
qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
lock->pending_writer--;
/* The rest of the write-side critical section is run with
* the mutex taken, so that lock->reader remains zero.
* There is no need to update self->locks_held.
*/
}
void qemu_co_rwlock_upgrade(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
qemu_co_mutex_lock(&lock->mutex);
assert(lock->reader > 0);
lock->reader--;
lock->pending_writer++;
while (lock->reader) {
qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
lock->pending_writer--;
/* The rest of the write-side critical section is run with
* the mutex taken, similar to qemu_co_rwlock_wrlock. Do
* not account for the lock twice in self->locks_held.
*/
self->locks_held--;
}
