block: Add bdrv_aio_cancel_async

This is the async version of bdrv_aio_cancel, which doesn't block the
caller. It guarantees that the cb is called either before returning or
some time later.

bdrv_aio_cancel can base on bdrv_aio_cancel_async, later we can convert
all .io_cancel implementations to .io_cancel_async, and the aio_poll is
the common logic. In the end, .io_cancel can be dropped.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -4640,7 +4640,32 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
 
 void bdrv_aio_cancel(BlockDriverAIOCB *acb)
 {
-    acb->aiocb_info->cancel(acb);
+    if (acb->aiocb_info->cancel) {
+        acb->aiocb_info->cancel(acb);
+    } else {
+        qemu_aio_ref(acb);
+        bdrv_aio_cancel_async(acb);
+        while (acb->refcnt > 1) {
+            if (acb->aiocb_info->get_aio_context) {
+                aio_poll(acb->aiocb_info->get_aio_context(acb), true);
+            } else if (acb->bs) {
+                aio_poll(bdrv_get_aio_context(acb->bs), true);
+            } else {
+                abort();
+            }
+        }
+        qemu_aio_release(acb);
+    }
+}
+
+/* Async version of aio cancel. The caller is not blocked if the acb implements
+ * cancel_async, otherwise we do nothing and let the request normally complete.
+ * In either case the completion callback must be called. */
+void bdrv_aio_cancel_async(BlockDriverAIOCB *acb)
+{
+    if (acb->aiocb_info->cancel_async) {
+        acb->aiocb_info->cancel_async(acb);
+    }
 }
 
 /**************************************************************/

include/block/aio.h

@@ -27,6 +27,8 @@ typedef void BlockDriverCompletionFunc(void *opaque, int ret);
 
 typedef struct AIOCBInfo {
     void (*cancel)(BlockDriverAIOCB *acb);
+    void (*cancel_async)(BlockDriverAIOCB *acb);
+    AioContext *(*get_aio_context)(BlockDriverAIOCB *acb);
     size_t aiocb_size;
 } AIOCBInfo;
 

include/block/block.h

@@ -337,6 +337,7 @@ BlockDriverAIOCB *bdrv_aio_discard(BlockDriverState *bs,
                                    int64_t sector_num, int nb_sectors,
                                    BlockDriverCompletionFunc *cb, void *opaque);
 void bdrv_aio_cancel(BlockDriverAIOCB *acb);
+void bdrv_aio_cancel_async(BlockDriverAIOCB *acb);
 
 typedef struct BlockRequest {
     /* Fields to be filled by multiwrite caller */