coroutine-lock: add qemu_co_rwlock_downgrade and qemu_co_rwlock_upgrade
These functions are more efficient in the presence of contention.
qemu_co_rwlock_downgrade also guarantees not to block, which may
be useful in some algorithms too.
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-3-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h
index a4509bd..9aff9a73 100644
--- a/include/qemu/coroutine.h
+++ b/include/qemu/coroutine.h
@@ -229,6 +229,24 @@
void qemu_co_rwlock_rdlock(CoRwlock *lock);
/**
+ * Write Locks the CoRwlock from a reader. This is a bit more efficient than
+ * @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
+ * However, if the lock cannot be upgraded immediately, control is transferred
+ * to the caller of the current coroutine. 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.
+ */
+void qemu_co_rwlock_upgrade(CoRwlock *lock);
+
+/**
+ * Downgrades a write-side critical section to a reader. Downgrading with
+ * @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
+ * followed by @qemu_co_rwlock_rdlock. This makes it more efficient, but
+ * may also sometimes be necessary for correctness.
+ */
+void qemu_co_rwlock_downgrade(CoRwlock *lock);
+
+/**
* Write Locks the mutex. If the lock cannot be taken immediately because
* of a parallel reader, control is transferred to the caller of the current
* coroutine.
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index b44b5d5..846ff91 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -402,6 +402,21 @@
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);
@@ -416,3 +431,23 @@
* 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--;
+}
