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>

include/qemu/coroutine.h

@@ -228,6 +228,24 @@ void qemu_co_rwlock_init(CoRwlock *lock);
  */
 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

util/qemu-coroutine-lock.c

@@ -402,6 +402,21 @@ void qemu_co_rwlock_unlock(CoRwlock *lock)
     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 @@ void qemu_co_rwlock_wrlock(CoRwlock *lock)
      * 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--;
+}