[Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback

[Kevin Wolf <kwolf@redhat.com>]

Request callbacks can do pretty much anything, including operations that
will yield from the coroutine (such as draining the backend). In that
case, a decreased in_flight would be visible to other code and could
lead to a drain completing while the callback hasn't actually completed
yet.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
---
 block/block-backend.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 1b2d7a6ff5..2efaf0c968 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1338,8 +1338,16 @@ static const AIOCBInfo blk_aio_em_aiocb_info = {
 static void blk_aio_complete(BlkAioEmAIOCB *acb)
 {
     if (acb->has_returned) {
-        blk_dec_in_flight(acb->rwco.blk);
+        if (qemu_get_current_aio_context() == qemu_get_aio_context()) {
+            /* If we are in the main thread, the callback is allowed to unref
+             * the BlockBackend, so we have to hold an additional reference */
+            blk_ref(acb->rwco.blk);
+        }
         acb->common.cb(acb->common.opaque, acb->rwco.ret);
+        blk_dec_in_flight(acb->rwco.blk);
+        if (qemu_get_current_aio_context() == qemu_get_aio_context()) {
+            blk_unref(acb->rwco.blk);
+        }
         qemu_aio_unref(acb);
     }
 }
-- 
2.13.6