block: Add refcnt in BlockDriverAIOCB

This will be useful in synchronous cancel emulation with
bdrv_aio_cancel_async.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -4891,13 +4891,23 @@ void *qemu_aio_get(const AIOCBInfo *aiocb_info, BlockDriverState *bs,
     acb->bs = bs;
     acb->cb = cb;
     acb->opaque = opaque;
+    acb->refcnt = 1;
     return acb;
 }
 
+void qemu_aio_ref(void *p)
+{
+    BlockDriverAIOCB *acb = p;
+    acb->refcnt++;
+}
+
 void qemu_aio_release(void *p)
 {
     BlockDriverAIOCB *acb = p;
-    g_slice_free1(acb->aiocb_info->aiocb_size, acb);
+    assert(acb->refcnt > 0);
+    if (--acb->refcnt == 0) {
+        g_slice_free1(acb->aiocb_info->aiocb_size, acb);
+    }
 }
 
 /**************************************************************/

include/block/aio.h

@@ -35,11 +35,13 @@ struct BlockDriverAIOCB {
     BlockDriverState *bs;
     BlockDriverCompletionFunc *cb;
     void *opaque;
+    int refcnt;
 };
 
 void *qemu_aio_get(const AIOCBInfo *aiocb_info, BlockDriverState *bs,
                    BlockDriverCompletionFunc *cb, void *opaque);
 void qemu_aio_release(void *p);
+void qemu_aio_ref(void *p);
 
 typedef struct AioHandler AioHandler;
 typedef void QEMUBHFunc(void *opaque);