qemu/util/qemu-coroutine-lock.c
Paolo Bonzini 050de36b13 coroutine-lock: Reimplement CoRwlock to fix downgrade bug
An invariant of the current rwlock is that if multiple coroutines hold a
reader lock, all must be runnable. The unlock implementation relies on
this, choosing to wake a single coroutine when the final read lock
holder exits the critical section, assuming that it will wake a
coroutine attempting to acquire a write lock.

The downgrade implementation violates this assumption by creating a
read lock owning coroutine that is exclusively runnable - any other
coroutines that are waiting to acquire a read lock are *not* made
runnable when the write lock holder converts its ownership to read
only.

More in general, the old implementation had lots of other fairness bugs.
The root cause of the bugs was that CoQueue would wake up readers even
if there were pending writers, and would wake up writers even if there
were readers.  In that case, the coroutine would go back to sleep *at
the end* of the CoQueue, losing its place at the head of the line.

To fix this, keep the queue of waiters explicitly in the CoRwlock
instead of using CoQueue, and store for each whether it is a
potential reader or a writer.  This way, downgrade can look at the
first queued coroutines and wake it only if it is a reader, causing
all other readers in line to be released in turn.

Reported-by: David Edmondson <david.edmondson@oracle.com>
Reviewed-by: David Edmondson <david.edmondson@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210325112941.365238-5-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2021-03-31 10:44:21 +01:00

468 lines
13 KiB
C

/*
* 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);
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);
}
struct CoRwTicket {
bool read;
Coroutine *co;
QSIMPLEQ_ENTRY(CoRwTicket) next;
};
void qemu_co_rwlock_init(CoRwlock *lock)
{
qemu_co_mutex_init(&lock->mutex);
lock->owners = 0;
QSIMPLEQ_INIT(&lock->tickets);
}
/* Releases the internal CoMutex. */
static void qemu_co_rwlock_maybe_wake_one(CoRwlock *lock)
{
CoRwTicket *tkt = QSIMPLEQ_FIRST(&lock->tickets);
Coroutine *co = NULL;
/*
* Setting lock->owners here prevents rdlock and wrlock from
* sneaking in between unlock and wake.
*/
if (tkt) {
if (tkt->read) {
if (lock->owners >= 0) {
lock->owners++;
co = tkt->co;
}
} else {
if (lock->owners == 0) {
lock->owners = -1;
co = tkt->co;
}
}
}
if (co) {
QSIMPLEQ_REMOVE_HEAD(&lock->tickets, next);
qemu_co_mutex_unlock(&lock->mutex);
aio_co_wake(co);
} else {
qemu_co_mutex_unlock(&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. */
if (lock->owners == 0 || (lock->owners > 0 && QSIMPLEQ_EMPTY(&lock->tickets))) {
lock->owners++;
qemu_co_mutex_unlock(&lock->mutex);
} else {
CoRwTicket my_ticket = { true, self };
QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
qemu_co_mutex_unlock(&lock->mutex);
qemu_coroutine_yield();
assert(lock->owners >= 1);
/* Possibly wake another reader, which will wake the next in line. */
qemu_co_mutex_lock(&lock->mutex);
qemu_co_rwlock_maybe_wake_one(lock);
}
self->locks_held++;
}
void qemu_co_rwlock_unlock(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
assert(qemu_in_coroutine());
self->locks_held--;
qemu_co_mutex_lock(&lock->mutex);
if (lock->owners > 0) {
lock->owners--;
} else {
assert(lock->owners == -1);
lock->owners = 0;
}
qemu_co_rwlock_maybe_wake_one(lock);
}
void qemu_co_rwlock_downgrade(CoRwlock *lock)
{
qemu_co_mutex_lock(&lock->mutex);
assert(lock->owners == -1);
lock->owners = 1;
/* Possibly wake another reader, which will wake the next in line. */
qemu_co_rwlock_maybe_wake_one(lock);
}
void qemu_co_rwlock_wrlock(CoRwlock *lock)
{
Coroutine *self = qemu_coroutine_self();
qemu_co_mutex_lock(&lock->mutex);
if (lock->owners == 0) {
lock->owners = -1;
qemu_co_mutex_unlock(&lock->mutex);
} else {
CoRwTicket my_ticket = { false, qemu_coroutine_self() };
QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
qemu_co_mutex_unlock(&lock->mutex);
qemu_coroutine_yield();
assert(lock->owners == -1);
}
self->locks_held++;
}
void qemu_co_rwlock_upgrade(CoRwlock *lock)
{
qemu_co_mutex_lock(&lock->mutex);
assert(lock->owners > 0);
/* For fairness, wait if a writer is in line. */
if (lock->owners == 1 && QSIMPLEQ_EMPTY(&lock->tickets)) {
lock->owners = -1;
qemu_co_mutex_unlock(&lock->mutex);
} else {
CoRwTicket my_ticket = { false, qemu_coroutine_self() };
lock->owners--;
QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
qemu_co_rwlock_maybe_wake_one(lock);
qemu_coroutine_yield();
assert(lock->owners == -1);
}
}