block: add aio_wait_bh_oneshot()

Sometimes it's necessary for the main loop thread to run a BH in an
IOThread and wait for its completion.  This primitive is useful during
startup/shutdown to synchronize and avoid race conditions.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20180307144205.20619-2-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/block/aio-wait.h

@@ -113,4 +113,17 @@ typedef struct {
  */
 void aio_wait_kick(AioWait *wait);
 
+/**
+ * aio_wait_bh_oneshot:
+ * @ctx: the aio context
+ * @cb: the BH callback function
+ * @opaque: user data for the BH callback function
+ *
+ * Run a BH in @ctx and wait for it to complete.
+ *
+ * Must be called from the main loop thread with @ctx acquired exactly once.
+ * Note that main loop event processing may occur.
+ */
+void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
+
 #endif /* QEMU_AIO_WAIT */

util/aio-wait.c

@@ -38,3 +38,34 @@ void aio_wait_kick(AioWait *wait)
         aio_bh_schedule_oneshot(qemu_get_aio_context(), dummy_bh_cb, NULL);
     }
 }
+
+typedef struct {
+    AioWait wait;
+    bool done;
+    QEMUBHFunc *cb;
+    void *opaque;
+} AioWaitBHData;
+
+/* Context: BH in IOThread */
+static void aio_wait_bh(void *opaque)
+{
+    AioWaitBHData *data = opaque;
+
+    data->cb(data->opaque);
+
+    data->done = true;
+    aio_wait_kick(&data->wait);
+}
+
+void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
+{
+    AioWaitBHData data = {
+        .cb = cb,
+        .opaque = opaque,
+    };
+
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+
+    aio_bh_schedule_oneshot(ctx, aio_wait_bh, &data);
+    AIO_WAIT_WHILE(&data.wait, ctx, !data.done);
+}