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From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: [Qemu-devel] [PULL v2 24/24] coroutine-lock: make CoRwlock
thread-safe and fair
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Cc: Peter Maydell <peter.maydell@linaro.org>,
Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
This adds a CoMutex around the existing CoQueue. Because the write-side
can just take CoMutex, the old "writer" field is not necessary anymore.
Instead of removing it altogether, count the number of pending writers
during a read-side critical section and forbid further readers from
entering.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213181244.16297-7-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
include/qemu/coroutine.h | 3 ++-
util/qemu-coroutine-lock.c | 35 ++++++++++++++++++++++++-----------
2 files changed, 26 insertions(+), 12 deletions(-)
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index d2de268..e60beaf 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -204,8 +204,9 @@ bool qemu_co_queue_empty(CoQueue *queue);
=20
=20
typedef struct CoRwlock {
- bool writer;
+ int pending_writer;
int reader;
+ CoMutex mutex;
CoQueue queue;
} CoRwlock;
=20
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index b0a554f..6328eed 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -346,16 +346,22 @@ void qemu_co_rwlock_init(CoRwlock *lock)
{
memset(lock, 0, sizeof(*lock));
qemu_co_queue_init(&lock->queue);
+ qemu_co_mutex_init(&lock->mutex);
}
=20
void qemu_co_rwlock_rdlock(CoRwlock *lock)
{
Coroutine *self =3D qemu_coroutine_self();
=20
- while (lock->writer) {
- qemu_co_queue_wait(&lock->queue, NULL);
+ 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++;
}
=20
@@ -364,10 +370,13 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
Coroutine *self =3D qemu_coroutine_self();
=20
assert(qemu_in_coroutine());
- if (lock->writer) {
- lock->writer =3D false;
+ 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 >=3D 0);
/* Wakeup only one waiting writer */
@@ -375,16 +384,20 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
qemu_co_queue_next(&lock->queue);
}
}
- self->locks_held--;
+ qemu_co_mutex_unlock(&lock->mutex);
}
=20
void qemu_co_rwlock_wrlock(CoRwlock *lock)
{
- Coroutine *self =3D qemu_coroutine_self();
-
- while (lock->writer || lock->reader) {
- qemu_co_queue_wait(&lock->queue, NULL);
+ qemu_co_mutex_lock(&lock->mutex);
+ lock->pending_writer++;
+ while (lock->reader) {
+ qemu_co_queue_wait(&lock->queue, &lock->mutex);
}
- lock->writer =3D true;
- self->locks_held++;
+ 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.
+ */
}
--=20
2.9.3