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Re: [PATCH 4/5] coroutine-lock: reimplement CoRwLock to fix downgrade bu
From: |
David Edmondson |
Subject: |
Re: [PATCH 4/5] coroutine-lock: reimplement CoRwLock to fix downgrade bug |
Date: |
Wed, 17 Mar 2021 10:40:42 +0000 |
On Tuesday, 2021-03-16 at 17:00:06 +01, Paolo Bonzini wrote:
> A feature 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.
>
> 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 if it is a reader, causing
> all other readers to be released in turn.
>
> Reported-by: David Edmondson <david.edmondson@oracle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> include/qemu/coroutine.h | 10 ++-
> util/qemu-coroutine-lock.c | 150 ++++++++++++++++++++++++-------------
> 2 files changed, 106 insertions(+), 54 deletions(-)
>
> diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
> index 84eab6e3bf..ae62d4bc8d 100644
> --- a/include/qemu/coroutine.h
> +++ b/include/qemu/coroutine.h
> @@ -237,11 +237,15 @@ bool qemu_co_enter_next_impl(CoQueue *queue,
> QemuLockable *lock);
> bool qemu_co_queue_empty(CoQueue *queue);
>
>
> +typedef struct CoRwTicket CoRwTicket;
> typedef struct CoRwlock {
> - int pending_writer;
> - int reader;
> CoMutex mutex;
> - CoQueue queue;
> +
> + /* Number of readers, of -1 if owned for writing. */
s/, of/, or/
> + int owner;
> +
> + /* Waiting coroutines. */
> + QSIMPLEQ_HEAD(, CoRwTicket) tickets;
> } CoRwlock;
Isn't this...
* ... Also, @qemu_co_rwlock_upgrade
* only overrides CoRwlock fairness if there are no concurrent readers, so
* another writer might run while @qemu_co_rwlock_upgrade blocks.
...now incorrect?
> /**
> diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
> index eb73cf11dc..655634d185 100644
> --- a/util/qemu-coroutine-lock.c
> +++ b/util/qemu-coroutine-lock.c
> @@ -327,11 +327,70 @@ void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex)
> 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)
> {
> - memset(lock, 0, sizeof(*lock));
> - qemu_co_queue_init(&lock->queue);
> qemu_co_mutex_init(&lock->mutex);
> + lock->owner = 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->owner here prevents rdlock and wrlock from
> + * sneaking in between unlock and wake.
> + */
> +
> + if (tkt) {
> + if (tkt->read) {
> + if (lock->owner >= 0) {
> + lock->owner++;
> + co = tkt->co;
> + }
> + } else {
> + if (lock->owner == 0) {
> + lock->owner = -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);
> + }
> +}
> +
> +/* Releases the internal CoMutex. */
> +static void qemu_co_rwlock_sleep(bool read, CoRwlock *lock)
> +{
> + CoRwTicket my_ticket = { read, qemu_coroutine_self() };
> +
> + QSIMPLEQ_INSERT_TAIL(&lock->tickets, &my_ticket, next);
> + qemu_co_mutex_unlock(&lock->mutex);
> + qemu_coroutine_yield();
> +
> + if (read) {
> + /* Possibly wake another reader, which will wake the next in line.
> */
> + assert(lock->owner >= 1);
> + qemu_co_mutex_lock(&lock->mutex);
> + qemu_co_rwlock_maybe_wake_one(lock);
> + } else {
> + assert(lock->owner == -1);
> + }
> }
>
> void qemu_co_rwlock_rdlock(CoRwlock *lock)
> @@ -339,13 +398,13 @@ void qemu_co_rwlock_rdlock(CoRwlock *lock)
>
> 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);
> + if (lock->owner == 0 || (lock->owner > 0 &&
> QSIMPLEQ_EMPTY(&lock->tickets))) {
> + lock->owner++;
> + qemu_co_mutex_unlock(&lock->mutex);
> + } else {
> + qemu_co_rwlock_sleep(true, lock);
> }
> - lock->reader++;
> - qemu_co_mutex_unlock(&lock->mutex);
>
> - /* The rest of the read-side critical section is run without the mutex.
> */
> self->locks_held++;
> }
>
> @@ -355,69 +413,58 @@ 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);
> + self->locks_held--;
> +
> + qemu_co_mutex_lock(&lock->mutex);
> + if (lock->owner == -1) {
> + lock->owner = 0;
> } else {
> - self->locks_held--;
> + lock->owner--;
> + }
>
> - 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);
> - }
> + if (lock->owner == 0) {
> + qemu_co_rwlock_maybe_wake_one(lock);
> + } else {
> + qemu_co_mutex_unlock(&lock->mutex);
> }
> - 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);
> + qemu_co_mutex_lock(&lock->mutex);
> + assert(lock->owner == -1);
> + lock->owner = 1;
>
> - /* The rest of the read-side critical section is run without the mutex.
> */
> - self->locks_held++;
> + /* 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);
> - lock->pending_writer++;
> - while (lock->reader) {
> - qemu_co_queue_wait(&lock->queue, &lock->mutex);
> + if (lock->owner == 0) {
> + lock->owner = -1;
> + qemu_co_mutex_unlock(&lock->mutex);
> + } else {
> + qemu_co_rwlock_sleep(false, lock);
> }
> - 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.
> - */
> + 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);
> + assert(lock->owner > 0);
> + /* For fairness, wait if a writer is in line. */
> + if (lock->owner == 1 && QSIMPLEQ_EMPTY(&lock->tickets)) {
> + lock->owner = -1;
> + qemu_co_mutex_unlock(&lock->mutex);
> + } else {
> + lock->owner--;
> + qemu_co_rwlock_sleep(false, lock);
Doesn't this need something for the case where lock->owner hits 0?
If not, how is two readers both attempting to upgrade ever resolved?
It feels like it should jump into the second half of
qemu_co_rwlock_wrlock().
> }
> - 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--;
> }
> --
> 2.29.2
dme.
--
And you're standing here beside me, I love the passing of time.
- [PATCH v3 0/5] coroutine rwlock downgrade fix, minor VDI changes, Paolo Bonzini, 2021/03/16
- [PATCH 5/5] test-coroutine: Add rwlock downgrade test, Paolo Bonzini, 2021/03/16
- [PATCH 4/5] coroutine-lock: reimplement CoRwLock to fix downgrade bug, Paolo Bonzini, 2021/03/16
- Re: [PATCH 4/5] coroutine-lock: reimplement CoRwLock to fix downgrade bug,
David Edmondson <=
- [PATCH 1/5] block/vdi: When writing new bmap entry fails, don't leak the buffer, Paolo Bonzini, 2021/03/16
- [PATCH 3/5] coroutine/mutex: Store the coroutine in the CoWaitRecord only once, Paolo Bonzini, 2021/03/16
- [PATCH 2/5] block/vdi: Don't assume that blocks are larger than VdiHeader, Paolo Bonzini, 2021/03/16